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ANNUAL REPORTS ON

NMR SPECTROSCOPY

This Page Intentionally Left Blank

ANNUAL REPORTS ON

NMR SPECTROSCOPY Edited by G . A. WEBB Department of Chemistry, University of Surrey, Guildford, Surrey, England

VOLUME 30

ACADEMIC PRESS Harcourt Brace & Company, Publishers London

San Diego Tokyo

0

Toronto

ACADEMIC PRESS LIMITED 24-28 Oval Road, LONDON NW17DX

U.S. Edition Published by

ACADEMIC PRESS INC. San Diego, CA 92101

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Copyright

01995 ACADEMIC PRESS LIMITED

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No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical including photocopying, recording, or any information storage and retrieval system without permission in writing from the publisher

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LIST OF CONTRIBUTORS

J. D. Augspurger, Department of Chemistry, Cornell University, Ithaca, New York 14853, USA.

P. J. Barrie, Department of Chemistry, University College London, 20 Gordon Street, London W C l H OAJ, UK.

M. BudCSinsky , Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic. C. E. Dykstra, Department of Chemistry, Indiana University - Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202, USA.

K. Kamienska-Trela, Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44, Warsaw 01-224, Poland. D. Saman, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic. K. Takegoshi, Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan.


There are no lacunae in molecular science as far as the fecund applications of NMR spectroscopy are concerned. NMR is now generally accepted as an exoteric technique by scientists from a broad variety of backgrounds; this is a trend which shows little sign of abating. Volume 30 of Annual Reports on N M R Spectroscopy consists of reviews which serve to exemplify this viewpoint. It is a pleasure to be able to present in the present volume accounts on Calculation and Prediction of Structural N M R Shifts in Respiratory Proteins by Professors J. D . Augspurger and C. E . Dykstra, N M R Applications to Porous Solids by D r P. J. Barrie, Miscibility, Morphology and Molecular Motion in Polymer Blends by Professor K. Takegoshi, One-Bond 13C--13C Spin-Spin Coupling Constants by D r K. Kamienska-Trela and 13C N M R Spectra of Sesquiterpene Lactones by Drs M. Bud5Sinskq and D . Saman. The variety of science covered in these reviews helps to demonstrate the widespread importance of NMR spectroscopy. Finally, I am very happy to express my gratitude to the production staff at Academic Press (London) for their unstinting assistance in the production of this volume.

University of Surrey Guildford, Surrey England

G. A. WEBB


Contents List of Contributors . . . . . . . . . . . . . Preface

.

. . . . . . . . . . . . . . . .

...

111

. v

Calculation and Prediction of Structural NMR Shifts in Respiratory Proteins J . D . AUGSPURGER and C . E . DYKSTRA 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . Theoretical Approach to NMR Parameters . Shielding Dependence on Protein Structure . Long-range Effects on NMR Parameters . . Property Correlations in Respiratory Proteins Acknowledgement . . . . . . . . . References . . . . . . . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

1 4 18 21 30 33 33

NMR Applications to Porous Solids P . J . BARRIE 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . NMR studies of microporous materials . . . . . NMR studies of mesoporous materials . . . . . '*'Xe NMR studies of porous materials . . . . . NMR studies of molecular transport in porous solids Acknowledgement . . . . . . . . . . . References . . . . . . . . . . . . . .

. . . . . 37 . . . . . 38

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

66 75 81 85 85

Miscibility. Morphology and Molecular Motion in Polymer Blends K . TAKEGOSHI 1. 2. 3. 4.

Introduction . Miscibility . . Polymer-polymer Morphology .

. . . . . . . . . . . . . . . . . 97 . . . . . . . . . . . . . . . . . 101 interaction . . . . . . . . . . . . . 110 . . . . . . . . . . . . . . . . . 115

5 . Molecular motion . Note added in proof Acknowledgement References . . .

1. 2. 3. 4.

5. 6. 7.

8. 9. 10. 11. 12. 13.

. . . . . . . . . . . . . . . . 122 . . . . . . . . . . . . . . . 126 . . . . . . . . . . . . . . . . 126 . . . . . . . . . . . . . . . . 126

One-bond 13C-13C Spin-Spin Coupling Constants K . KAMIENSKA-TRELA Introduction . . . . . . . . . . . . . . . . Theoretical considerations . . . . . . . . . . . . Unsubstituted hydrocarbons . . . . . . . . . . . Substituent effects on one-bond CC spin-spin couplings across single, double and triple bonds . . . . . . . . . One-bond CC spin-spin coupling constants in derivatives of benzene . . . . . . . . . . . . . . . . . . . One-bond CC coupling constants in heteroaromatic systems . One-bond CC couplings in substituted aliphatic cyclic and heterocyclic systems . . . . . . . . . . . . . The lone pair effect . . . . . . . . . . . . . One-bond CC couplings in structural studies of complexes . One-bond CC couplings in charged molecules and some related compounds . . . . . . . . . . . . . . One-bond CC couplings in biological studies . . . . . . Experimental methods . . . . . . . . . . . . . Application of the INADEQUATE method in structural . . . . . . . . . . . . . . . . elucidations Note added in proof . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

. . 131 . . 132 . . 140 .

. 144

. 153 . . 157

. . 161 . . 180 .

. 186

. . 196 . . 200 . . 212

. . 216 . . 219 . . 222

Carbon-13 NMR Spectra of Sesquiterpene Lactones M . BUDESINSKY and D . SAMAN 1. 2. 3. 4.

Introduction . . . . . . . . Structure classification . . . . . Methods of structure determination . Carbon-13 NMR spectra . . . . References . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . .

232 232 235 242 456

Index . . . . . . . . . . . . . . . . . . . . . 477 Cumulative Indexes for Authors and Subjects. Volumes 21-30 . . . 485

Calculation and Prediction of Structural NMR Shifts in Respiratory Proteins J. D. AUGSPURGER* and C. E. DYKSTRAT *Department of Chemistry, Cornell University, Ithaca, New York 14853, USA ?Department of Chemistry, Indiana University-Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202, USA 1 4 4 7 9 10 18 18 19 19 21 21 26 29 30 33 33

1. Introduction 2. Theoretical approach to NMR parameters 2.1. Energy derivatives 2.2. Magnetic property operators 2.3. Choice of gauge 2.4. A b initio approaches and basis sets 3. Shielding dependence on protein structure 3.1. Variation with torsions 3.2. Variation with bond lengths 3.3. Dynamical variation 4. Long-range effects on NMR parameters 4.1. Chemical shielding 4.2. Nuclear quadrupole coupling 4.3. Spin-spin coupling 5. Property correlations in respiratory proteins Acknowledgement References

1. INTRODUCTION In the past decade, NMR has become a significant tool in the determination of three-dimensional protein structures, driven mainly by the advances in multi-dimensional NMR techniques.' These techniques exploit the nuclear Overhauser effect (NOE) to extract distances between particular nuclei.* These distances can then be used as constraints in conjunction with energy minimization to determine native protein structure in solution (e.g. refer-

ence 3). The ability to relate specific chemical shifts t o molecular structure was one focus of a recent NATO Advanced Research W o r k ~ h o p .This ~ capability ANNUAL REPORTS ON NMR SPECTROSCOPY VOLUME 30 ISBN 0-12-505330-4

Copyright 01995 Academic Press Limited AN rights of reproduction in any form reserved

2

J. D . AUGSPURGER AND C. E. DYKSTRA

could provide information that would significantly expand the structural information which can be extracted from NMR data. This review surveys efforts toward that goal, and describes the current state of the art in calculating chemical shifts in macromolecules. Definition of terms is a proper place to begin. The chemical shielding, (T, is formally the second derivative of the molecular eigenenergy with respect to an applied, uniform magnetic field and the size of the nuclear magnetic moment. Its absolute value is not usually measured. Instead, the chemical shift, 6, is the measured shielding referenced to a particular shielding standard. For half a century, the sensitivity of the shielding and the shift to the type of chemical bonding at a given nucleus has been used extensively. There is also sensitivity to the entire molecular environment that comes about in other ways. For proteins, we shall refer to one of those ways as a “structural shift”, meaning the difference between the chemical shift of a particular nucleus in a folded (native) protein and the chemical shift of the same nucleus in the unfolded (denatured) protein. The latter is the random coil protein structure: (1) Random coil refers to a flexible, extended state of the protein where essentially all values of local conformational angles, $C and $, are sampled (i.e. averaged over). Thus, these properties are solely dependent on the primary structure of tne protein. For example, any amide proton of a leucine residue in a random coil protein would be expected to have nearly the same chemical shift. The random coil shifts of the protons of all 20 naturally occurring amino acids have been reported by Wuthrich and coworker^.^,^ Various approaches have been taken to calculate structural shifts in macromolecules. Early work in this area concerned predicting the contribution of ring currents to chemical shifts in nucleic acids.’ The theory of ring current contributions to chemical shifts and its application has been reviewed by Haigh and Mallion.8 Subsequent work included the contribution due to the magnetic anisotropy of the peptide carbonyl group.’ Buckingham first described the direct effect of electrical polarization on chemical shifts,” and this contribution was included in the description of structural shifts. The work on nucleic acids was reviewed extensively by Giessner-Prettre and Pullman in 1987.l2 Similar approaches have been less frequently applied to peptides and proteins: Sternlicht and Wilson13 included the effects of ring currents and electrical polarization in analysing the structural shifts in lyzosyme. Asakura er al. examined the changes in the chemical shifts of poly-L-alanine due to the helix to coil transition, concluding that changes from solvent effects are more significant than those due to conformation. l4 Perkins and Wuthrich compared the calculated ring current shifts to the sstruct

=

sobserved

- 6random

coil

STRUCTURAL NMR SHIFTS IN RESPIRATORY PROTEINS

3

experimentally observed structural shifts for bovine pancreatic trypsin inhibitor (BPTI)." They found that the ring currents were the dominant influence on the structural shifts for the peripheral side chain protons, but were not dominant for the backbone protons (amide and Ca>. Hoch et ~ 1 . ' ~ used molecular dynamics (MD) simulations to study dynamical influences on the ring current shifts in BPTI, and found that the fluctuations were sometimes large, up to +6ppm. However, the time averaged values were close to those of the averaged structure. Clayden and Williams17 attempted unsuccessfully to explain the experimentally observed geminal a-CH2 inequivalence via magnetic anisotropy and electrical polarization. More recently, work has been focused on finding empirical relations for experimentally observed chemical shifts in proteins of known structure. To date, the chemical shifts of about 120 proteins have been completely identified. '' There have been several reports concerned with cataloguing these data, and seeking empirical correlations. 19-*' Wagner et ~ 1 . ~reported " correlations between the structural shifts of the amide (NH) and C" protons and the inverse third power of the distance between the proton and the nearest carbonyl oxygen. Strong correlation has been reported between secondary structure and the structural shifts: upfield shifts of about 0.35 ppm in helices and downfield shifts of about 0.38 ppm for NH and C" proton^.^^-^^ Williamson and AsakuraZ5 empirically fit the C" proton chemical shifts of nine proteins with well-defined structures using a 10 parameter model based on astruct = aring current

+ arnagnetic anisotropy + aelectric

(2)

Osapay and Case26 catalogued the proton shifts of 17 proteins, whose structures had been identified via X-ray crystallography. They sought to fit the data empirically, adding a term to equation (2) to include solvent effects. They found that ring current effects alone do not represent the structural shifts accurately for C" protons, but that using the additional terms in equation (2) resulted in correlation coefficients of 0.85 or better for all except the NH protons. Wishart et a1.27have carried out the most extensive cataloguing of peptide and protein chemical shifts, collecting the shifts from 78 peptides and proteins, including 4888 NH, 5134 C", and 13 933 side chain proton resonances. In their analysis, besides the previously observed upfield/ downfield correlations with helix/sheet structures, they also observed that NH protons near the N-terminal end of the helix are shifted downfield relative to those near the C-terminal end. Spera and Bax" have observed similar types of correlations for 13C resonances from a database of 442 shifts. C" I3C nuclei are shifted upfield in helices and downfield in beta sheets, while Cp carbons exhibit the opposite

4

J. D. AUGSPURGER AND C. E. DYKSTRA

trend. The shifts of each group vary by about 6-8 ppm, compared to ranges of 1-2 ppm for protons. Oldfield et pioneered 170NMR in proteins, observing essentially linear correlations between 13C and 170 chemical shifts and vco IR frequencies in carbonmonoxyheme proteins. A molecular model based on intermolecular electrical polarization was shown by Augspurger et ~ 1 to . produce semiquantitative agreement with the observed correlations. Further, the range of structural shifts of various nuclei were shown to correlate with representative values of the shielding polarizabilities of the nuclei, indicating the contribution of polarization is likely to dominate for nuclei other than protons.32 19Fshifts, for instance, in "F-tryptophan labelled hen egg-white lysozyme were reproduced quite a c ~ u r a t e l y . ~ ~ As Oldfield has done,33 one of the simplest steps in analysing structural shifts is to regard them as arising from long- and short-range effects: ~

1

.

~

~

3

~

"

= @short + @long (3) then represents that part of the chemical shielding due to the local electron density at the nucleus as dictated by local structure (c$,I,!J). represents the long-range or interresidue interactions of the protein, as well as solvent interactions. Oldfield and coworkers have used ab initio calculations on representative molecular fragments to obtain @short and have incorporated long-range electrical effects by including corresponding point charges for the other atoms in the protein, including dynamical effects, through molecular dynamics (MD) sir nu la ti on^.^^-^^ @

@short

2. THEORETICAL APPROACH TO NMR PARAMETERS

Theoretical methods have been developed and have been available for over 20 years for the direct calculation of the chemical shielding tensor. For the most part, ab initio methods for such calculation have been limited to molecules that are quite small relative to proteins. However, this technology is advancing, and it is likely that many useful applications for protein structure will be developed in the years ahead. 2.1. Energy derivatives

From the standpoint of the quantum mechanical electronic structure of a molecule, the NMR chemical shielding is simply one of many intrinsic properties, each defined as a specific derivative of the molecular electronic energy. Energy derivatives may be calculated in several ways. One way is by taking finite differences. Since the energy is to be differentiated with respect to some parameter embedded in the Hamiltonian, then a set of calculations can be carried out to find the energy for various choices of the parameter's

~

~


5

value. If two such calculations are done, an approximate value of the first derivative is the difference in two energies divided by the difference in the parameter's values. The two possible difficulties in a finite difference procedure both have to do with the size of the parameter value increment. It may be so small that the energy difference in the numerator is of the size of the numerical accuracy of the energy values, and so the derivative value would not be reliable. Or, the parameter increment may be so large that the energy difference between two points is contaminated by higher derivatives. Both these potential difficulties are controllable and d o not have to interfere with the quality of the results, but considerable effort may have to be invested to be sure that the final results are, in fact, free of such numerical inaccuracies. A second way of obtaining first derivative properties is through computing an expectation value. A dipole moment, for instance, may be calculated by integrating the system's probability density function with the dipole moment operator. That result will be equivalent to the derivative of the energy if the wavefunction obeys the Hellmann-Feynman t h e ~ r e m ~ ' .or, ~ ' in general, if the wavefunction is completely variationally determined. Another means of obtaining properties is by analytical differentiation, the underlying idea being rigorous differentiation and subsequent solution of the Schrodinger equation. Differentiation is with respect to parameters embedded in the Hamiltonian. Within the Born-Oppenheimer approximation, for instance, the nuclear positions are parameters. Parameters may also correspond to some perturbation, and so field strength will be a parameter for any property that is defined as a derivative with respect to the field. Thus, the Hamiltonian will depend on the choice of parameters; that is, H = H ( a , b , c , . . .). At this point, we will take all Hamiltonian parameters to be independent. A specific choice of the parameters implies a specific Schrodinger equation. It is convenient to refer to some reference choice or zeroth order choice of the parameters and, in this discussion, that reference specification will be all parameters at zero. A zero subscript (e.g. a. or Ho) will refer to invoking this specification. Thus, from the general time-independent Schrodinger equation ( H - E)+ = 0, there comes a specific equation for solution (H" - E"M0 = 0

(4)

The task of differentiating the Schrodinger equation begins with formal differentiation of the Hamiltonian and then setting parameters to their reference value. Derivatives of the energy at the reference parameter choice are properties. The derivatives with respect to some parameter a of the Hamiltonian operator and of the wavefunction will be designated with superscripts:

6


First order differentiation of the Schrodinger equation yields a simple expression:

Ha$+ H$"

=

Ea++ E$"

(6)

With the reference choice of parameters, this becomes

H; *o+Ho

G = -% *o+Eo G

(7)

Solving this expression requires the zero order Schrodinger equation solutions, and this is analogous to perturbation theory. Indeed, the general procedure differs from perturbation theory in only one minor way. A perturbation expansion is in terms of the powers of the parameters, whereas the derivative expansion is a Taylor series expansion that has each nth power series term divided by n ! relative to the perturbation expansion. Equation (7) may be rearranged as

E: *o

=

(Ho - Eo)

+ H: *o

*?I

(8)

Integration with the zero order wavefunction yields an expression for the first derivative of the energy: E?l = ( *o I Ho - Eo I *o) + (*o I H : I *o)

(9)

If the zero order wavefunction is variational, then the first term in equation (9) is identically zero, and then the first derivative is simply an expectation value of the derivative Hamiltonian. This is one expression of the Hellmann-Feynman theorem. Rearrangement of equation (6) to collect terms involving the unknown derivative function yields the following: ( H - E)@

=

- ( H a - E")+

(10)

All higher derivative equations can be put into this same form, ( H -E)$" = X , where X is some function of wavefunctions and energies of derivative order less than a. Solving this type of equation is not significantly more complicated than solving the zero order Schrodinger equation, and the generality in form means that solutions can be found order-by-order to any level. Likewise, higher order derivatives of the energy are obtained, and this can be carried out for differentiation with respect to one or many parameters. As in perturbation theory, there is a "2n + 1 rule" applicable in electronic structure3w1, and it means that from the nth order wavefunctions, energy derivatives of order up to 2n + 1 may be evaluated. The derivative wavefunctions between n and 2n + 1 orders are not required explicitly. For example, with the first derivative wavefunctions known explicitly, the third energy derivatives may be evaluated immediately.


7

2.2. Magnetic property operators To carry out an ab initio calculation of an energy derivative (property), we require derivative operators, e.g. H". For NMR shieldings, the interaction term in the molecular Hamiltonian arises from the external field and the nuclear magnetic dipole. The strength of the field and the strength of the magnetic dipole are the parameters, and differentiation is carried out with respect to these. We may also be interested in the change in the shielding with respect to another influence, perhaps an electric field. Electric field strength is then also a parameter. The Hamiltonian for a molecule experiencing an external magnetic field depends on the vector potential, A, of that magnetic field: .

2rn

V is the usual potential energy operator in the molecular Hamiltonian and pi is the vector momentum operator for electron i, rn is the mass of an electron, e is the electron charge and c is the speed of light. Notice that whereas the Hamiltonian explicitly depends on the vector potential, A, not the magnetic field, B, the chemical shielding (T is a derivative with respect to B , not A. This problem can be surmounted in two ways. One way is for the Hamiltonian to be rewritten to depend explicitly on B, by writing A in terms of B. For a uniform magnetic field, this is accomplished by 1 A = -BXr 2

(12)

where r is the position vector. The other way is for the derivatives with respect to B to be written in terms of derivatives with respect to A.42 While this second approach is more general, the first is normally followed. The magnetic field of the nuclear magnetic dipole, for which the vector potential is

(where rN is the nuclear position vector), must be added to that of the external magnetic field, and the sum substituted into equation (11). The necessary derivative operators for calculating (T are

ifie 2rnc

- -(r x

V),

8

J . D. AUGSPURGER AND C. E. DYKSTRA

and

Note that these relationships hold for the Coulomb gauge, i.e. where V * A = 0. If a further perturbation by an external electric field or field gradient is included, the perturbing Hamiltonian will include field and field gradient terms, and these may be collected into a dot product: Helec =

M .V

Here, M is a vector composed of the electrical moment operators of the molecule, and V is a vector composed of derivatives of the external electrical potential (evaluated at the same centre as the molecule's electrical moments). The derivative of the Hamiltonian with respect to a particular electric field component is just the corresponding molecular moment operator. For instance, the derivative Hamiltonian operator with respect to the x-component of the gradient of the potential (the negative of the electric field in the x-direction) is the x-component of the dipole moment. To carry out a calculation of the molecular wavefunction, energy and properties where the wavefunction is represented by a linear combination of atomic orbitals, the matrix representation of the Hamiltonian and its

Table 1. Classification of (equation algorithm."

(18)) operators for general construction

Operator type or kind

Ikl

1'1

(1'4

Iml

Basis function overlap operator Kinetic energy operator Nuclear attraction Electric field, gradients Electrical potential interaction (multipole moments); magnetic field interaction (second order part) Magnetic field interaction (first order part)' Dipolar magnetic field interaction (first part)

0 0 0

0 0 0

0 2 0

0 20

>0 0

Without Without With With Without

0 0

0 0 0 0 30

20

0

Without

1

0

0

1

With

1

0

"/PI

In1

is the sum of the three integers in the set (px, p,., p z } ; (p represents k , I, rn or n). "This is for the coulomb gauge, where V . A = 0. Otherwise, another term is required where / k / = 0 and l n / 3 0 .


9

derivatives in the atomic orbital basis are required as input. A general approach for calculating all such one-electron integrals has been presented.43 This algorithm, implemented in the program MAGOPS,& calculates the matrix representation of the operator

where specification of any particular operator is made by specification of the superscript integers, represented in vector notation by (k,l,m,n). Table 1 presents several operators specified in this manner. A uniform approach for computing matrix representations of this operator, without limit as to types of Gaussian basis functions ( . ~ , p , d , f., . .) or the choice of the (k,l,m,n) integers has been implemented and is in use.43

2.3. Choice of gauge

Any vector Y for which V X Y = 0 may be added to the vector potential A without changing the magnetic field B. Of course, any such change in the vector potential must leave the magnetic field response properties unaffected. This arbitrariness in the vector potential is an arbitrariness in the choice of the gauge. Because of the basis set truncation in an ab initio calculation with conventional orbital functions, magnetic properties are not strictly invariant to the choice of this gauge. This is particularly problematic for small basis sets. D i t ~ h f i e l destablished ~~,~~ the use of orbitals that were dependent on A in such a way that the final results were independent of the choice of the gauge. These have been designated as gauge-invariant atomic orbitals (GIAO), though it is not the atomic orbitals that are invariant but the magnetic properties. Ditchfield has pointed out that “gauge-dependent’’ is a better way of describing these orbitals,47 and it has become customary to take GIAO to mean “gauge-including atomic orbital^".^^ GIAOs are constructed from conventional, atom-centred basis functions by choosing a coordinate system origin for the species under study and incorporating an origin-dependent function. Let %, be the position vector from that origin to the a-nucleus. For a uniform magnetic field, B, a G I A O basis function, $(r), at the centre a may be defined in terms of a conventional Gaussian function centred on the a nucleus, X(r):

4(r) = X(r) exp( - i B X R, r/2)

(19)

10


The GIAOs have a complex exponential dependence on the part of the vector potential arising from the external uniform magnetic field, and one may substitute A,,,

1 2

= -BX

R,

(20)

for the direct product in equation (20). The form of GIAOs leads to oneand two-electron integrals that are dependent on the external magnetic field. The one-electron current density associated with a wavefunction $ is defined49as j(r) =

1 L1

($*V$- $V$*) - A$*$/c

and should be gauge invariant and conserved. Epstein” has shown that use of GIAOs does not ensure current conservation because the GIAO wavefunction is not invariant to a gauge transformation. The gauge invariance is termed an “enforced” invariance because the gauge dependence of the GIAOs amounts to changing the basis to follow a gauge change, “or equivalently, by always returning to the original gauge to do the c a l c ~ l a t i o n ” .The ~ ~ GIAOs may be thought of as providing basis set flexibility that would be found only in larger conventional sets. The usefulness of GIAOs is something mainly demonstrated by calculational results.

2.4. Ab initio approaches and basis sets The primary distinction among ab initio approaches is whether they include or do not include correlation effects. The majority of calculational results available now are at the self-consistent field or SCF (uncorrelated) level. Fortunately, SCF appears to be well suited in many (not all) of the shielding problems of light elements in covalently bonded molecules. A second distinction among methods is whether the shieldings are obtained analytically or by finite fields. Mostly, this difference has implications on the cost of the computation more than on the results. A further distinction is the treatment of gauge dependence. Table 2 lists the identifiers now in use for ab initio approaches along with information on the type of calculation. The earliest work on methods for calculating derivatives of the SCF energy was based on perturbative approaches, which became known as “coupled-perturbed Hartree-Fock” (CPHF) .67-69 Gerratt and Mills7’ generalized the formulation of Stevens et ~ 1so that . ~ derivatives ~ with respect to basis-dependent parameters could be obtained with CPHF. This advance


11

Table 2. Designations for various ab initio methods/programs for calculating

chemical shifts and related properties. Designation

Wavefunction

DHF IGLO LORG DOGON TEXAS90 SOPPA SOLO GIAO-MP2

SCF SCFIMCSCF SCF RPA SCF MP2 MP2 MP2 MP2

-

Gauge treatment CommodGIAO Separate origin for each MO Local origin Local origin GIAO basis Common origin Same as LORG GIAO basis Common origin

Reference 41, 51 52-54 55, 56 57-59 60 61 62, 63 64,65 66

made possible Ditchfield’s use of G I A O to calculate chemical shieldings via CPHF.45“7 Kutzelnigg and ~ o - w o r k e r s ~have ~ - ~significantly ~ generalized the use of gauge-dependent orbitals for the calculation of NMR parameters. Instead of a common origin for & in equation (20), they use the centroid of charge of each different occupied, localized orbital, applying the same type of exponential involving the vector potential, but to the molecular orbitals. That is, the gauge is independent for each localized orbital (IGLO). Though additional terms must be computed, only conventional two-electron integrals involving the original basis functions are required. Applications of the IGLO method have already been reviewed.71 Hansen and B ~ u r n a n ’ ~have .~~ applied a conceptually similar local orbitalAoca1 origin (LORG) approach to the random phase approximation. Lazzeretti and ~ o - w o r k e r s ~have ~-~~ likewise introduced a multiple-origin gauge method, this one based on computing susceptibilities from nuclear electric shieldings which they term “distributed origin gauge with origin at the nuclei (DOGON)”.59Pulay and co-workers have recently reported a new, highly efficient implementation of the GIAO approach to calculate chemical shieldings, which takes advantage of certain recent advances in ab initio technology.@ The efficiency of this program, TEXAS90, makes possible calculations on larger systems, such as benzylideneaniline and retinylideneb~tylimime.~~ All of these approaches involve analytically solving the first order CPHF equations to calculate the chemical shielding. Via the CPHF equations,

where D is the one-electron density matrix and h is the matrix representation of the one-electron part of the Hamiltonian and its derivatives. Dykstra and J a ~ i e n , ~following ’ the derivative formalism of Section 2.1, devised a

12

J. D . AUGSPURGER AND C. E. DYKSTRA

procedure for solving derivative Hartree-Fock equations uniformly to all orders of differentiation. This approach is identified simply as derivative Hartree-Fock (DHF) theory. It was initially implemented to calculate non-magnetic properties, but has been extended to handle complex operators and thereby obtain magnetic proper tie^.^^ This method, by virtue of its generality, can yield not only electrical and magnetic properties, but mixed electro-magnetic properties. This capability has been exploited to calculate the direct influence on chemical shifts of external electrical perturbation^.^"^^'^^^^ These properties have been termed shielding polarizabilities because they measure the induced chemical shielding due to an external electrical influence:

DHF has also been used to explore how nuclear quadrupole coupling constants are influenced electr~statically.~~ Accurate calculation of molecular properties requires careful selection of the appropriate basis set. Chesnut’s 1989 review dealt with this topic e ~ t e n s i v e l y In . ~ ~his review of basis set tests carried out for the various methods described, he reports that the common gauge origin CPHF (or DHF), IGLO and GIAO methods require, respectively, [7s5pld/ 5slp],[5~4pld/3slp],and [4s3pld/2s] basis sets to calculate (T accurately for first row atoms and hydrogen. Second row atoms require even more extensive basis sets.79This assessment, of course, is subject to the limits one chooses for considering a value to be “accurate”. If basis sets were enlarged step-by-step beyond these three specific sizes toward a complete basis limit, gauge dependence in the non-GIAO treatments would necessarily diminish. Eventually, the basis requirements for the different treatments would be similar. So, the choice of gauge-including versus gauge-independent bases is a practical one: clearly, if a small basis is to be used, it is better that it be gauge-including. However, for limiting case studies (extended basis sets), there is a sizeable additional computational cost associated with the gauge-including bases that may shift the practical choice the other way. Apparent advantages of using gauge-including bases may be partly offset by computational costs. The IGLO method requires the calculation of a number of one-electron operators not required in a standard, fieldindependent basis calculation. Using a GIAO basis adds the further complication of additional derivative two-electron integrals. In effect, by incorporating the gauge into the basis set, differentiation generates derivative basis functions of higher angular momentum, creating a more flexible basis from a smaller one. A recent idea is that of Chesnut who demonstrated the use of “locally dense basis sets”.80The idea is based on (T being primarily dependent on the


13

local wavefunction, and so a larger, more flexible basis is used for the atom for which IT is to be calculated than for the other atoms in the molecule. It has shown promise in initial tests," and has been applied by Hinton et a1." to a system of 17 water molecules. We have recently carried out extensive basis set tests of not only u,but also the shielding polarizability, for carbon monoxide73 and The gauge sensitivity was also examined as a function of basis. For CO, we calculated these properties using DHF with a number of basis sets, ranging from double zeta to an extended set that was triple zeta, tripIy polarized with diffuse valence functions and a 4f function, while the common gauge origin was varied along the entire length of the molecular axis. These results are shown in Figs 1-3. Following the Kutzelnigg and Schindler result that extra flexibility in the p-functions is important, we compared the standard [5p/2p] and [6p/3p] Dunning c o n t r a c t i ~ nof~ ~the ~ ~Huzinaga ~ primitive basess4 to less contracted sets, [5p/4p] and [6p/5p]. In all cases, these less contracted bases yielded significantly less gauge dependence. We found little difference between [5p/4pJ and [6p/5p]. It is significant that for the 1 7 0 shielding, even unpolarized basis sets give shielding which are quite insensitive to the gauge origin, but which are 100 to 200 ppm lower than the larger basis results. In other words, gauge invariance does not go hand-inhand with basis quality and reliability. In our study of water,74 we examined the sensitivity of the proton isotropic and anisotropic chemical shielding. At the reference equilibrium geometry, the proton isotropic shielding and anisotropy were calculated, first with the gauge origin chosen to be the hydrogen centre and then with it chosen to be the oxygen nucleus. Comparison of the two largest basis results (Fig. 1) with the oxygen nucleus as the gauge centre shows a difference in isotropic shielding of 0.02ppm and a difference in the anisotropy of -0.05 ppm, demonstrating basis set convergence. Next, for the largest basis, we see that there is only 0.43 pprn difference between choosing the oxygen atom to be the gauge centre versus the hydrogen (30.86 with the gauge origin at the hydrogen versus 30.43 ppm for the gauge origin at the oxygen). A similar difference of 0.61 pprn is obtained for the anisotropy, and to this extent, gauge invariance has been achieved. Comparison of GI A O results shows the success of that approach, since a 6-311G basis gives very nearly the same result as our largest basis [10~7p5d3f/6~6p3d] result for the isotropic shielding. However, for the anisotropy, G I A O calculations show a lingering basis set dependence that matches our oxygen gauge origin calculations. In other words, the anisotropy is more demanding of basis set quality than the isotropic shielding, independent of gauge considerations. Likewise, the use of the less contracted oxygen p function set shows significant improvement in reducing the gauge dependence of the isotropic shielding, but not the anisotropy. It is interesting to see that choosing the gauge centre at the oxygen yields

14

J . D. AUGSPURGER AND C. E. DYKSTRA

the best agreement between small and large basis set results. This is not surprising in view of the successes of the IGLO In IGLO calculations, molecular orbitals are localized and a gauge origin for each orbital is placed at the orbital’s centroid of charge. In water, the centroid of the total electronic charge is very near the oxygen atom, and so the common origin calculations we have performed with oxygen as the gauge centre almost amount to invoking the ideas of IGLO. (For molecules with more than one heavy centre, this correspondence of approaches does not exist.) So, with the oxygen centre as the gauge origin, even the relatively small DZP basis gives an isotropic shielding and anisotropy which is within 1ppm of the Basis I11 result. It appears that a [7s7p4d/4s3p] basis is sufficiently flexible to achieve results within 0.5 pprn of the basis set limit with the gauge origin at the oxygen centre. While most calculations of chemical shielding have been at the SCF level, recently several methods have been reported for incorporating electron correlation. As the diamagnetic contribution is merely an expectation value, it can readily be calculated for any method of incorporating correlation. The complication arises from the paramagnetic term, which requires the first order response of the correlated wavefunction to the external magnetic field. Oddershede and Geertsen included correlation by using the polarization propagator approach to calculate chemical shieldings.61 They called this approach SOPPA (second order polarization propagator approximation). Bouman and H a n ~ e n ~utilized * > ~ ~ a similar approach to include correlation within their LORG algorithm, which was designated SOLO (second order LORG). They have used it to examine correlation effects in 31P shifts for a number of small molecules,62 and have looked at several six-member heteroatom ring systems, and benzene.63 They find that the isotropic shift is consistently greater with inclusion of correlation, up to 50 pprn for ”N and 10 ppm for I3C. Conversely, the anisotropy is consistently less. Gauss has developed a method for calculating chemical shieldings by analytical differentiation of the MP2 energy, using GIAO bases. The method is designated GIAO-MP2,64,65and it has been included in the ACES11 suite of a6 initio programs.*’ Cybulski and Bishop have also implemented analytical differentiation of the MP2 energy to calculate magnetic properties,66 using a relaxed density formalism.

Fig. 1. The proton (a) isotropic shielding (5= 1/3[c, + ayy+ a,,) and (b) shielding where a33> azz> a l l ) of water plotted as a anisotropy (Au = 1/2[2u33 - azz- all], function of basis set, comparing the choice of gauge origins to be at the proton (0)or at the oxygen nucleus (m). GIAO results are indicated by s. Solid lines indicate the valence basis was a standard D Z or TZ set, whereas broken lines indicate the less contracted sets, e.g. DZ’ and TZ’. The 4-31G basis result is from ref. 85, and the other GIAO results are unpublished results of deDios and Oldfield.86

I


90 80 70

60

50 40 30

45

15

16

J . D. AUGSPURGER A N D C. E. DYKSTRA

-150 ?

I

cd

-200

W

6 a

a+,

A -250

h

b -300

t

-3501. -0.5 0.0 1

-275

-325 h

I

-

cd

W

.

1

,

0.5

1

.

8

1

.

1.5

1.0

.

I

2.5

2.0

4 7 :

(b)

TZ+3Pi DZP

-375-

2 -4251 TZ

-475

DZ' TZ'

t

DZ

-5251' -0.5 0.0 1

.

8

0.5

.

1

1.0

.

8

1.5

.

8

.

2.0

8

2.5

Gauge Origin (a.u.) Fig. 2. cr\,,,.(ppm) for (a) I3C and (b) "0 as a function of gauge origin along the molecular axis (x-axis) for different basis sets. On the scale of the graph for I3C, the results for the TZP and TZ+3P bases are essentially coincident, as are those of TZ'P, TZ'2P, and TZ'+3P with TZ'+3Pf. For I7O, the DZP basis result is essentially coincident with TZ+3P, as are TZ'2P and TZ'+3P with TZ'+3Pf. The nearly coincident curves are not displayed. Broken lines are used for clarity only. The carbon atom lies at the origin, the centre of mass is at x = 1.218, and the oxygen atom is at x = 2.132.


17

-

-200 h

I

cd -400 -

v

E a a

-600 -

h

2-800 -1000

t

-12001 -0.5

"

0.0

.

1

'

'

.

I

.

I

.

I

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

1.5

2.0

3600.

c

=I

TZ

3200

r5

-

DZ TZ: DZ

.

DZP

v

E a a h h X

2800

26-

2400

TZP

TZ+3P TZ P DZ'P TZ+3Pf

-

TZ'2P

2000 I

1

-0.5

.

2.5

Gauge Origin (a.u.) Fig. 3. A,,,, (pp,m/a.u.) for (a) 13C and (b) " 0 as a function of gauge origin along the molecular axis (x-axis). For 13C,the DZ' basis result is essentially coincident with TZ', DZ'P results are essentially coincident with TZ'P results, and TZ'2P and TZ'+3P results with TZ'+3Pf results. For "0, the TZ'+3P and TZ'+3Pf bases are essentially coincident. The nearly coincident curves are not displayed. The carbon atom lies at the origin, the centre of mass is at x = 1.218, and the oxygen atom is at x = 2.132.

18


The accuracy of calculated chemical shieldings values is such that they are being used as independent checks of three dimensional structure in small to medium sized molecules.88 This level of accuracy was largely based on a study of 21 boranes and carboranes," where IGLO was employed to calculate the chemical shifts using MP2/6-31G* optimized molecular geometries. The standard deviation of the calculated "B chemical shifts versus experimental results was 3.1 ppm for all 21 compounds. However, for two compounds, 1,5-C2B3H5 and 1,2-C2B3H7, the differences were 10 and 8ppm respectively. The chemical shifts of these two compounds have subsequently been studied with the inclusion of electron c o r r e l a t i ~ nThe .~~ boron shifts in these compounds were calculated via the GIAO-MP2 method,64,6sand the differences with experiment were less than 1ppm. 3. SHIELDING DEPENDENCE ON PROTEIN STRUCTURE

Chemical shielding is most strongly dependent on the local electron density, due to the r-3 dependence of the perturbing Hamiltonian operators (see equations (16) and (17)). Small changes in electronic structure are readily manifested in the chemical shielding. These small changes which result from non-covalent interactions, such as electrical perturbation, ring currents and magnetic anisotropy , can be described via classical approaches. These long-range influences will be discussed in the next section. However, variation in the covalent bonding, such as changes in bond lengths and bond angles, must be described quantum mechanically.

3.1. Variation with torsions As it is clearly not possible to carry out rigorous, fully a b initio calculations on proteins, the approach taken must be to carry out calculations on model . recently ~ ~ carried out peptide or peptide-like fragments. deDios et ~ 1 have such a study of the dependence of 13C and "N chemical shifts in model peptides. To model the 13C chemical shifts of C" and C p , they chose terminally blocked alanine. By varying the 4 and CC, backbone angles, they find similar ranges of variation in the 13C shift as seen experimentally.28 They then calculated the C" and C p shifts of this model amino acid for the 12 alanines in staphylococcal nuclease (Snase), using the local structures as determined by X-ray crystallography, obtaining reasonably good agreement with experiment. In further tests, they find that these intra-molecular influences are dominant for C" and C p shifts. They then examined 15N shifts, using an alanine-valine dipeptide as the model fragment. When o-(N) was plotted as a function of the sidechain angle, ,yl, it exhibited a maximum for the


19

staggered conformationand a minimum for the eclipsed conformation. The shift varied by about 20 ppm over the entire range of xl. Again, reasonably good agreement was found when the amide 15N shifts of valine in Snase were calculated using this model dipeptide. Bartield and co-workers have also looked at the torsion angle dependence of u.91They studied the terminally blocked Gly-Gly dipeptide, mapping out u as a function of the 4 and +b angles. Their results reproduced the experimental trends of C" and C p chemical shifts in helices and sheets. While there are many factors influencing chemical shifts, calculations such as these may be useful in understanding conformationally strained, small peptides.

3.2. Variation with bond lengths Chesnut has carefully reviewed calculations that show the variation in the shielding with bond length.78 The first derivative of the shielding with respect to a geometrical parameter, u',can be readily c a l c ~ l a t e d and ,~~~~~ there have been examinations of basis set effects for small molecules. Using 6-311G* basis for first row and [66 211(s)/6211(p)/ll(d)] for second row atoms, Chesnut has presented the following c o n c l ~ s i o n s : ~ ~ (1) ur is less than zero for stretching a heavy atom-heavy atom bond;

(2)

ur varies

for hydrogen-heavy atom stretches;

(3) u' is mostly due to paramagnetic contributions; (4) for hydrides, u' decreases, then increases going across the periodic table.

An indirect indication of the significance of the dependence of u on bond length is the finding that chemical shieldings calculated from theoretically optimized geometries show better agreement with experiment, than when experimental geometries are used.94 Chesnut and Wright9' carried out calculations of ur and w'' (d2u/dR2)for a large number of small molecules, and found that both are typically less than zero. For multiply bonded atoms, d correlates with u,but not for singly bonded atoms.

3.3. Dynamical variation

Dynamical effects on chemical shieldings become an important theoretical concern once a meaningful level of accuracy has been achieved for the shieldings of static structures. That seems to be the situation of contemporary ab initio methodology, and so there are some exciting attempts to

20

J . D . AUGSPURGER AND C. E. DYKSTRA

incorporate dynamical effects. In spite of the newness of some developments, the problem is an old one. Buckingham outlined the effects of diatomic vibration on NMR shieldings three decades ago.96 Dynamical treatments are mostly of two types, quantum mechanical and classical. The classical treatment is that of molecular dynamics (MD) simulation wherein the forces acting on each atom according to some chosen force field are integrated through small time steps to yield instantaneous velocities and displacements. For sufficiently small time steps, this exactly follows Newtonian laws that give the spatial positions of the atomic particles as a function of time. A time-average of any property can be obtained provided that the property of interest is known for the structure of the system at each of the numerous time steps. Quantum mechanical treatments tend to be limited to smaller systems than MD simulation because of the computational cost. A variety of techniques may be used, but there is a requirement that is in common with that of MD simulations: to obtain an on-average property, a property surface (the dependence of the property value on the structure) must be explicitly or implicitly available. In protein NMR, the important dynamical questions concern protein molecule dynamics and solvent dynamics. The chemical shielding surface information necessary for obtaining average shieldings, quantum mechanically or classically, is mostly the variation due to long-range effects. As discussed in the next section, techniques have been devised to model these effects in a manner that makes possible rapid computation for different structures (i.e. an implicit property surface). Franken97 has carried out a preliminary quantum mechanical study that hints at the nature of dynamical effects in a solvent, water. Using diffusion quantum Monte Carlo (QMC)98-100 implemented for the intermolecular vibrations among rigid molecules, 101,102 a rigorous average of proton shielding in the water dimer has been obtained. The water-water interaction averaged over the ground vibrational state of the isolated water dimer was found to change the proton shielding anisotropy by 3.3 ppm. Oldfield, Warshel and c o - w o r k e r ~have ~ ~ , ~used ~ classical MD simulations to calculate averaged 19Fchemical shifts of the five "F-labelled tryptophan (Trp) residues in galactose binding protein (GBP) from E. c01i.'03 Since fluorine is singly bonded, the short-range (quantum mechanical) changes in the chemical shift will likely be small, and thus, the structural shifts should be closely represented by the average electrical perturbation:

AAVJ +

+A y y V y y ) +

(24) Several simulations were carried out over 20 ps trajectories, using a modified version of the program ENZYMIX.lo4 This program uses a local reaction field approach to calculate the long-range electrostatic force^,"^ and explicitly incorporates electrical polarizability for some atoms. The results b " C t

=

~ X X ( V X J

~22(V22)


21

showed good agreement for the structural shifts, or relative differences for the five 19Fresonances in GBP. The 19F in TrpZs4,which is the most solvent exposed Trp residue, exhibited the largest fluctuations in Sstruct,indication that solvent interaction may be a dominant influence on chemical shifts. Gregory and Gerig1O6 have also examined the I9F chemical shifts in a "F-labelled fragment of the S-peptide. They used a semi-empirical approach to calculate S from ring current and van der Waals contributions. Their results showed that 6 calculated for static, energy-minimized structures were significantly different from the averaged values of the MD simulations.

4. LONG-RANGE EFFECTS ON NMR PARAMETERS

Long-range effects are those that arise in the absence of real chemical bonding interaction between the perturbing species and the given magnetic centre. They develop because of a change in the local electronic structure at the magnetic centre as a result of the perturbation. This change should be regarded as a very small change, but of course, one of the wonderful aspects of NMR is its unique sensitivity to small electronic structure differences. To understand the long-range effects, let us consider each type of NMR parameter.

4.1. Chemical shielding

There have been a number of recent attempts to rationalize the chemical shifts observed in proteins in structural terms, with most emphasis being placed on analysing 'H shift^.^^-^^ For 'H NMR, moderately good agreement between experimental and predicted chemical shifts based on known structures can be obtained by using random coil shifts, computed ring current effects, magnetic anisotropy, and electrical polarization.26 However, there has been much less progress in interpreting I3C, I5N and "F shifts, which are very much larger. Since ring current and magnetic anisotropy effects will be of the same magnitude, it seems unlikely that they will be as important for heavy nuclei as they are important in 'H NMR. Thus, it appears that the long-range contributions to these large variations in structural shifts for heavy nuclei must originate from electrical perturbation. To quantify the direct influence of electrical polarization on chemical shielding we may begin by formally expanding the chemical shielding as a power series in the electric field, as Buckingham has done:" 1 2

uaP= c & + A E , + - B E ~ + .

..

(25)

Table 3. Representative System

H-H IJ-C84C

8-CF3 H-CN H-CCH H3-CCN H(styrenes) H2S H~CCHZ HhCh H20

HOCH3 H3N

HF IJCl HBr

HI H-N(Uraci1) CH4

CO HzCO HzCCHz H-CCH

A,(ppmla.u. field)" 50.3,' 38.9,' 24.8,'77,' 70,' 50.5,d 49.4' 30,' 48,' 50,' 65' 45.1,'46.3' 50' 54.1' 67.2' 51,' 58' 53' 450," 69.9 64.4g 87.5' 47.3,' 47.3' 430" 27.7,' 26.6 81.5,' 45,' 71.5,' 79.4,d 79.1,'83.9 570,' 680,' 117.9 1100,' 110.71 1571

90" 0.0 374.5,' 376.9,d 393.6' 697.4,' 769.0k 1144.5k 733.9,' 750.7h

A,,A,,,and A,

h) h)

shielding polarizabilities.

Ax,x(ppmla. u. field2)"

A,(pprn/a.

189.7,' 185.89,d 191.26'

5.8'

114.4,b 43.4 234.9' 173.2' 13.6'

2.1' 89.5' 109.5'

190.8,' 39.q 80.4,' 62.9 278.3,' 277.4 329.9,' 325.0,d 269.2,e 314.g

3.5' 18.1' 61.8'

100,' 344.d 469.7' -359.7,' 535.5,' 3874.9,' -434.2k 1106.9,'

-269.6k 448.2,d -286.4' 3955.0k

532.7' 746.0'

1166.2k

351.8'

u.

efg)"

645.0h 422.6,h 428.6k 215.9fi 631.2k -30.0k 222.Ok 2093.7' 50.8' 1051.7,d 777.0' 1910.1,h 1927.0' 902.8* 401.1,' 381.6' 1526.7,' 1561.5,d 1242.4' 7018.9,' 6555.3' 3 195.7g 636.5,' 704,' 605.2," 597.1,d 490.2," 585.5' 551.4: 792," 505.2' 18843 1955.5* 1984.9 -181.5' 534.1' 1149.8'

1502.9,' 1489.6k -41 1.6k -238.4k - 1383.6k 12 991.8' 1613.4' 2761.2,d 1593.6' 11 336.1,h 11 334.0' 4339.3,' 4094.0' 5906.1,' 5937.6,d 1080" 70 843.0.' 47 862.0' 8486.1,h 1930,' 1621," 7760,d 6122,' 7674.8' 10 686.6'

512.1'

333.3h 603.6' 190.8' 1044.0' 1377.8' 741.7' 4319.3'

15 659.6' -5355.6' -11 364.6'

"Units are in ppm/a.u. field, (field)2 or field gradient. The xx-component of the field gradient corresponds to the xx-element of the Cartesian quadrupole moment (Mxx= 112 C,q,xT). The x-axis was generally chosen to be the axis of highest symmetry. The convention for this table is that implicit in equations (25)-(28). 'Reference 41. Tabulated in reference 110. "Reference 111. 'Reference 112. These values include electron correlation at the MP2 level. 'Reference 114. KReference 32. "Reference 113. 'Reference 107. 'Reference 109. 'Reference 115. 'Reference 116. "'Reference 108. N

w

24

J . D . AUGSPURGER AND C. E. DYKSTRA

The coefficient A in equation (25) can be identified as the x component of the dipole shielding polarizability, A,,,,, from equation (23). Likewise, B is the dipole shielding hyperpolarizability , B,,x,,, . Similarly, one can define the effect of a field gradient as the quadrupole shielding polarizability, e.g. A,,,, , analogous to the electrical quadrupole polarizability. These properties are derivatives, and can be obtained by the methods outlined earlier. As these properties are tensors, a somewhat more simplified representation can be found by defining isotropic shielding polarizabilities:

Table 3 shows calculated values of the shielding polarizabilities in the literature. Bishop and Cybulski have recently reported the first shielding polarizability values to include electron correlation. '12 Their results indicate that correlation effects at the MP2 level are larger for heavy atoms than for hydrogen. If the direct influence of electrical fieldsheld gradients is the main long-range contribution to structural shifts, it seems likely that there should be a correlation between the magnitude of the shielding polarizabilities of these heavy atoms, and the range of variation in their structural shifts. That is, as there is likely to be a maximum electric field attainable at any particular nucleus in a protein, the maximum variation in structural shifts should be proportional to this field value times the shielding polarizability of the particular nucleus. In Fig. 4, representative A, values are plotted versus the observed shift ranges for 'H, 13C (aromatic), 13C0, C170 and 1YF 30,103,117-120 These results can be fit to a linear equation, assuming the dominance of the dipole shielding polarizability: A8(ppm) = 0.00578 A,

+ 1.13

(29)

The slope in this equation, AVx = 0.006 a.u. (or -3 x lo7Vcm-'), represents the range of effective fields experienced by the nuclei in question, and it appears to be reasonable value, based upon previous electrostatic calculations of electric fields in proteins. Recently, we used a charge field perturbation approach to explore the effects of electrostatic influence on chemical shielding of 19F in different chemical environment^.^^ A conventional ab initio calculation was carried out to obtain the absolute shielding, and then included in the molecular


5

10

15

25

20

Shift Ranges (ppm) Fig. 4. Graph showing relationship between the isotropic dipole shielding polarizabilities, A, computed for prototype small molecules using derivative Hartree-Fock theory, and the observed chemical shift ranges, for (left to right) 'H, 13C (C-0), 13C (aromatic), "O (C-0) and "F nuclei in proteins.

Hamiltonian was a particular electrostatic perturbation. Since dipoles are the longest range contributor to the electrostatic potential of a neighbouring neutral molecule or molecule fragment, our charge field perturbation calculations were carried out with an ideal dipole in a chosen position in the vicinity of the molecule. Comparison with the calculated value in the absence of a dipole provides the relative chemical shielding, a shift, due to the dipole. We used a dipole of l.Oa.u. (2.54 D) as representative of local bond dipoles that may exist in bio-organic molecules. The dipoles were placed at a number of distances from the fluorine atom of the molecule being studied and were aligned with the axis of fluorine's chemical bond. The molecules that were studied are a sampling of different bonding environments for fluorine in hydrocarbons. Of course, since fluorine forms only single bonds, the dif€erences in the environments are secondary, at the connected centre. An analysis of the shifts due to electrostatic interaction comes about from partitioning the shielding values into the diamagnetic and paramagnetic contributions. The shift range due to electrostatic interaction shows very good correlation with the paramagnetic shielding, whereas the shift range and diamagnetic shieldings turn out to be uncorrelated. From these results, it is clear that the greater the paramagnetic shielding, the proportionately greater is the response to an external dipole. Since the paramagnetic shielding diminishes the overall shielding ( a = vdiag uPara, gars < 0), then

.

+

26


there is an anti-correlation between the total shielding and the shift range to the extent to which the diagmagnetic contribution is either unchanging or changing in parallel.

4.2. Nuclear quadrupole coupling Interaction of an electric quadrupole moment of a nucleus with an electric field gradient of the electron cloud is a small perturbation of the molecular energy, but it is measurable via nuclear quadrupole resonance, nuclear magnetic resonance, and also microwave spectroscopy. 12’ The existence of an electric field gradient requires a non-spherical charge distribution such as that arising when chemical bonding distorts the spherical symmetry of the electron distribution around a nucleus. There may be small field gradient changes that result from external or long-range perturbations. The original picture of variations in nuclear quadrupole coupling goes back to a series of paper^'^^-'^^ wherein Sternheimer presented an analysis of the effect of an external point charge on the electron distribution’s field gradient at a nucleus. This gave, as well, the extent to which a nuclear quadrupole polarizes an isolated atom’s electron distribution since, as shown,’23 it was equivalent to take the perturbation as an external charge or as a nuclear quadrupole. The phenomenological outcome of this analysis was the idea that the field gradient at a nucleus, V,,, can be taken to be a “shielded” external field gradient:

v,, = V,=X‘(1 - y )

(30)

where y is the Sternheimer shielding factor. Sternheimer shielding is sometimes introduced with a factor (1 + y ) instead of (1 - y), making for an opposite sign of y. Unlike chemical shieldings, IM does not generally turn out to be much less than 1.0. Sternheimer analysis has been extended to ionic diatomic molecules125to include the added effect of the induced dipole moment, and Buckingham’26 devised an expression to include explicitly the effects of uniform fields on ions, in effect, adding a term onto equation (31):

v,, = V;Tt

(1 - y ) + &(V,ext)2 (31) where E is a parameter. Engstrom er a1.I2’ applied these ideas to covalent molecules, using a perturbation approach. A different truncation of effects, V,, = V::‘

(1- 7)

+F Vyt

(32) was employed by Legon and Millen’2s~’29for microwave data on a series of HCI complexes. The most general analytical approach to nuclear quadrupole coupling constants, or electric field gradients at a nucleus, is to employ a power series expansion in terms of all the elements of the external electrical potential,


27

which has already been pointed out and considered by Baker et al. 130 Taking V F as the electric field gradient at a particular nucleus, an expansion in terms of external field components V,, V,, V , (suppressing the "ext" superscript used above), external field gradient components, V,,, Vxy,and so on, and higher tensor components is

v,",""= v2; + vg: v, + vp;v, + v::; v, + 21 vg;,v',, + . . . (33) The parameters in equation (33) (e.g. V,",::) are response properties of second and higher order. When the interaction with the nuclear quadrupole moment is included in the molecular Hamiltonian, these parameters are simply derivatives of the molecular eigenenergy to second and higher order. Bacskay et al. 131,132 and F o ~ l e r ' ~ ha ~ ve , ' ~used ~ finite field approaches to calculate certain of the terms in equation (33), and we have developed a fully analytical ab initio approach77 for all the response properties in equation (33). A difficulty in using these field derivatives of V z u c is the question of convergence in the multipole expansion. The contributions due to field gradients, hypergradients, second hypergradients, etc., do not necessarily decrease uniformly, as has been found by Baker et ~ 1 . ; ' ~they " calculated the terms in equation (33) through Vr",lt,,, (the first order response of V,","' to the second electrical hypergradient) for the 35Cl nucleus in HCI. Another question which arises is the origin dependence of these field derivatives. For a molecule with multiple heavy atoms, like HCN, the properties, and their relative contributions to V z " " , will be quite different if the centre of the expansion of the external field gradients is taken to be the centre of mass of HCN, as opposed to the 14N nucleus. Baker et ~ 1 . ' ~found " that inclusion of the first order terms in equation (33) through the second hypergradient was necessary to agree with finite perturbation calculations, where the partner molecule was represented by its low order multipole moments. These difficulties may be examined by reformulating the problem. Instead of asking how the field, field gradient, and so on, of a partner molecule influence the field gradient at the nucleus, we may ask: how does an ideal dipole, quadrupole, octupole, etc. at a given location directly influence the field gradient at the nucleus? This eliminates the question of origin dependence, as there is no Taylor series expansion of the external electrical potential to carry out. This leads to a new form of equation (33), where

28


Table 4. Generalized linear and non-linear Sternheimer shieldings for 14N in HCN and 35Clin HCI (a.u.).

Property"

HCN, origin

92 922

9xx 9221

9zxx 9z.z 92.22

92 ,xx 9z.rzz 92,ZXX

922.22 9ZZJX

9zz.zzz 9zz,zxx

9 x x Jix 9xx.y.v 9xx.zzz

9xx.zxx 9XX.ZYY

9zzz.zzz 9zzz.zxx 9zxx.zxx 9zxX.zYy

HCl, origin

centre of mass

I4N

7.126 4.229 -1.966 6.206 2.868 10.25 46.50 4.984 77.20 -2.924 63.99 5.173 145.4 24.23 -29.44 18.93 13.23 17.43 8.036 327.3 19.97 -10.95 -4.807

7.126 -3.785 -1.966 5.957 3.605 10.25 34.97 4.990 31.39 -4.792 -27.62 -0.432 67.57 27.68 -29.43 -18.93 10.57 28.47 15.13 116.7 -13.08 -28.16 -13.49

3 5 ~ 1

17.40 25.05 -16.67 33.38 -0.367 67.35 98.35 -3.718 290.3 -23.96 242.3 -46.54 413.3 -6.845 -222.7 -94.90 -0.139 - 10.28 2.560 1038 33.40 -54.45 - 16.98

O q Z = VY:; and so on (see equation 33).

Table 5. Moment shieldings of I4N in HCN (a.u.). R(A)b

Property"

3.0 qM,i

qM. qMz. qM:::

qM2;,:

qMrrr, qMU,,,,

3.5

4.0

4.5

5.0

6.0

7.0

0.277 23 0.093 31 0.063 32 0.045 74 0.200 53 0.150 58 0.116 91 -0.10020 -0.064 74 -0.042 83 -0.029 49 -0.021 09 -0.011 83 -0.007 27 0.007 10 0.003 30 0.001 73 0.049 58 0.030 19 0.017 96 0.011 05 -0.022 39 -0.016 79 -0.009 62 -0.005 43 -0.003 17 -0.001 22 -0.000 55 -0.004 42 0.009 01 0.005 89 0.003 21 0.001 74 0.000 56 0.000 22 0.010 37 0.005 44 0.002 57 0.001 27 0.000 66 0.000 21 0.000 08 -0.012 58 -0.000 93 0.000 37 0,000 34 O.OO0 21 0.000 07 0.000 03

''qMU,,= VY:; and so on (see equation 34). Only symmetry-unique, non-zero properties listed.

Due to Laplace's condition, several elements are related by the traceless properties of the and qM,,,, = - %4MzZz:. electric field (hyper)gradient tensors: qMx = - % q M M , , q M X M , ;=, - '/2qMz2, 'The moment properties given are calculated with respect to moments located at a distance R along the positive z-axis where the HCN centre of mass is at the origin and the I4N nucleus is on the positive z-axis.

29


Table 6. Moment shieldings of 3sCl in HCI (a.u.).

R(AIb

Property” 3.0

3.5

4.0

4.5

5.0

6.0

7.0

1.260 98 0.820 17 0.571 88 0.420 28 0.321 43 0.20498 0.141 80 -0.628 35 -0.340 52 -0.200 80 -0.127 36 -0.085 55 -0.043 81 -0.025 30 0.452 15 0.207 64 0.104 27 0.057 25 0.033 84 0.013 94 0.00673 -0,38951 -0.161 78 -0.070 85 -0.033 93 -0.017 70 -0.005 88 -0.002 38 0.363 53 0.147 73 0.058 67 0.024 81 0.011 48 0.003 09 0.001 04 0.212 71 0.064 96 0.023 20 0.009 49 0.004 34 0.001 16 0.OOO 39 0.006 14 0.002 92 0.001 40 0.000 39 0.000 13 -0.030 95 0.008 90

= VyJC, and so on (see equation 34). Only symmetry-unique, non-zero properties listed. Due to Laplace’s condition, several elements are related by the traceless properties of the electric field (hyper)gradient tensors: qicl, = - ~ q M z ,qMxu,,,= -%qw,,, and qMM,,,= -%qMzZz,. bThe moment properties given are calculated with respect to moments located at a distance R along the positive z-axis where the HCI centre of mass is at the origin and the 35Clnucleus is on the negative z-axis. uqM,

( M o , M , , M,,, represent components of the charge, first moment, and second moment, etc.) All the response properties in both equations (33) and (34) have been calculated for the heavy nuclei of HCN and HCl via DHF. These results77 are reproduced in Tables 4-6.

4.3. Spin-spin coupling Spin-spin coupling tensors have proven to be the most challenging NMR parameters to obtain from ab initio calculations. The coupling arises through the interaction of two nuclear magnetic moments with the electronic environment. There are interaction terms in the Hamiltonian for each nuclear magnetic source, and the spin-spin couplings are second derivatives with respect to the magnetic moments of two different nuclei. Calculations by Lazzeretti and ~ o - w o r k e r shave ~ ~ ~shown , ~ ~ the ~ major importance of the Fermi contact portion of the interaction in isotropic spin-spin couplings. Sekino and Bartlett137 carried out extensive correlated calculations on the very simple molecule H D . Their results suggest that somewhat unlike chemical shieldings, electron correlation effects tend to be much more important than inclusion of polarization functions in the basis set. In fact, it is often presumed that correlation effects are essential for meaningful evaluation of spin-spin coupling; of course, such rules-of-thumb tend to be broken somewhere. A study of several organic molecules by Laaksonen et al. 13’ indicates that correlation effects on spin-spin coupling are primarily those associated with dynamical versus non-dynamical correlation. Also important, is that agreement between correlated evaluations and

30


experimental values for H H and 13CH coupling in several small organic molecules was mostly better than 10 Hz. Spin-spin coupling may be regarded as arising both through space and through the electronic structure of the molecular framework. The Contributions from Localized Orbitals within the Polarization Propagator Approach (CLOPPA) method139 has been employed to distinguish contributions. For instance, in calculations on methylenedioxyben~ene,~~~ CH coupling was found to increase by several hertz because of the proximity of an aromatic CH bond cis to the oxygen lone pair. CH coupling appears to be a sensitive probe of intramolecular hydrogen bonding.'417142In proteins, the influence of intramolecular hydrogen bonding and solvent effects on spin-spin coupling are becoming fertile ground for ab initio calculations, given that it is likely that prototype or fragment molecular species calculations will provide suitable information.

5. PROPERTY CORRELATIONS IN RESPIRATORY PROTEINS

Almost any correlation of chemical shieldings with other molecular properties is of potential value in molecular structure elucidation, in analysing electronic structure reorganization, and in establishing certain structure/ function relationships. We have found that the long-range influences on chemical shielding in respiratory proteins can be correlated with a number of other proper tie^.^' The first correlation involves the C O in carbonmonoxyheme proteins. This correlation was based on the ideas of long-range effects in the last section, and so the first step was determining response properties of the C O group. A vibrational potential for unperturbed or isolated C O was obtained'43 from large basis set calculations using well-correlated, coupled cluster wavefunctions. 144-146Electrical and magnetic properties of C O were analytically calculated as a function of the bond distance, r, via DHF.41,51The basis set for these calculations was a large, 96-function set of contracted Gaussians. The chemical shielding tensors at the oxygen and carbon centres were evaluated along with shielding polarizabilities and nuclear quadrupole couplings. Vibrational wavefunctions for CO were calculated by the NumerovCooley rnethodl4' which provides an exact numerical determination of the vibrational energies and wavefunctions. This was done for C O experiencing various types of external electrical environments. The first and simplest type was a uniform electric field. The second type was a field gradient (only). The third type was the potential due to a positive ( + e ) and a negative ( - e ) charge arranged as a dipole, oriented either along or perpendicular to the C O axis. The charges were separated by 0.1 a.u. and their centre was at least 4 A from the CO multipole centre. These are four hypothetical

STRUCTURAL NMR SHIFTS I N RESPIRATORY PROTEINS

31

electrical perturbations of the CO molecule that correspond in a rough way to the types of electrical perturbations that proximate molecules or ligand groups might generate. For these same electrical environments, the effects on the chemical shieldings of carbon and oxygen were obtained via the shielding polarizabilities. The results showed a nearly linear relation between the isotropic chemical shifts and the vibrational frequencies that are in good, semi-quantitative accord with the trends found for C O in variously perturbed heme proteins .30 In particular, the opposite trends in 13C and 170chemical shifts and "0 nuclear quadrupole coupling constants with changes in C O vibrational frequency, usually described in terms of "backbonding", appear to be well predicted solely on the basis of electrical polarization. Figure 5 shows the net relation between the vibrational frequency and the chemical shifts for three different types of electrical environments that we examined, that of a uniform field, a field gradient and an axial point-charge dipole together with experimental I3C and I7O isotropic chemical shieldings and vibrational frequencies for a range of carbonmonoxyheme proteins .30 This shows the opposite pattern for 13C and 170,and this is mainly because of the shielding polarizability properties of CO. Since the electrical perturbation arising from almost any molecular charge distribution will have a uniform field component, and since this seems to be the important and dominant long-range influence on the chemical shieldings and vibrational frequencies, then this correlation should hold for many other systems. Finally, an additional set of ab initio calculations was done to find the nuclear quadrupole coupling constant of 170,and its correlation with the vibrational frequency. Electronic structure calculations determined the electric field gradient at the nucleus as a function of the bond length, r , and of the strength of the external axial electric field. Using a value of -2.64 fm2 for the nuclear quadrupole mornent,l4' we obtained the quadrupole coupling constant, e2qQlh. Then, the vibrationally averaged value, (e2qQ/h), for the ground vibrational state was found as a function of the external field. (The behaviour of the electric field gradient at the nucleus as a function of the vibrational coordinate has already been considered in detail in ab initio studies of Cummins et a[.148 and Amos.149) From this, we obtain the correlation of (e2qQlh)with vibrational frequency shift, which agrees nicely with the experimental data for h e~ n e - C O . ~ ' In addition to the correlation of properties among various carbon monoxyheme proteins brought about through common electrical perturbation, Oldfield et al. have identifiedI5' a correlation of the relative conformational substate energies with the electrical perturbation source. For a wide variety of heme proteins, including the haemoglobins, myoglobins and several peroxidases, the vibrational frequency shifts can be related to electric field differences arising from 180" ring flips of the Ha and Ha tautomers of distal histidine residues.

32


60 20

40

t

0 4,

0 20

-20

0

** t

-40

-20

-60

t

40

-100

0

1

-60

-40

-20

0

-40

-20

20

40

60

10

0

-10

-20

-60

O

20

A vo1 Fig. 5. Plots of the isotropic chemical shifts for 1 7 0 and for 13C versus the change in the fundamental vibrational transition frequency, calculated for (a) various uniform electric fields, (b) various field gradients and (c) various positions of an axial dipole consisting of two point charges, and (d) measured experimentally for a range of carbonmonoxyheme protein^.^' For the experimental values, the carbonmonoxyheme proteins (from ref. 30) were: Glyceru dibrunchiatu haemoglobin, picket fence porphyrin, haemoglobin Zurich, Physete cutudon myoglobin (pH = 7.0, 7.2 and “low”), rabbit haemoglobin (a-chain), rabbit haemoglobin (/.?-chain), human adult haemoglobin (a-chain), human adult haemoglobin (/.?-chain), lactoperoxidase, horseradish peroxidase isoenzyme A (pH = 9.5, 6.8 and 4.5) and horseradish


33

The correlation of chemical shieldings, other properties and relative energetics via electrical influence suggests that important structural information can be extracted from NMR shieldings of proteins provided there is available information on shielding polarizabilities. The ultimate role of ab initio calculations in protein NMR is in the evaluation of these properties and, more broadly, in the attempt to determine a local or relative effect on a shift. Valuable information can be obtained short of an ab initio calculation on an entire protein. This conclusion opens the door to further connections between ab initio studies and NMR experiments.

ACKNOWLEDGEMENT

This work was supported, in part, by a grant from the United States National Institutes of Health (Grant No. 19481).

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peroxidase isoenzyme B (pH = 10.5, 7 and 6.4); these are referenced to adult haemoglobin. The solid lines represent the straight line, least squares fit of the data. The slope of the line for 13C is -0.07ppm cm-' and the corresponding slope from the calculations in (a) is -0.23 ppm cm-'. The slope of the line for "0 is 0.26 ppm cm-' and the corresponding slope from the calculations in (a) is 0.47 ppm cm-'.

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35

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36


110. W. T. Raynes, in Specialist Periodical Reports (ed. R. K. Harris), Vol. 1, 36. Chemical Society, London, 1972. 111. D. M. Bishop and S. M. Cybulski, Mol. Phys., 1993, 80, 199. 112. D. M. Bishop and S. M. Cybulski, Mol. Phys., 1993, 80, 209. 113. J. D. Augspurger and C. E. Dykstra, Mol. Phys., 1992, 76, 229. 114. M. Grayson and W. T. Raynes, Chem. Phys. Lett., 1994, 218, 270. 115. M. Grayson and W. T. Raynes, Chem. Phys. Lett., 1993, 214, 473. 116. M. Grayson and W. T. Raynes, Mol. Phys., 1994, 81, 533. 117. A. Allerhand, R. F. Childers and E. Oldfield, Biochemistry, 1973, 12, 1335. 118. E. Oldfield and A. Allerhand, J . Biol. Chem., 1975, 250, 6403. 119. W. E. Hull and B. D. Sykes, J . Mof. Biol., 1975, 98, 121. 120. M. P. Gamcsik, J . T. Gerig and R. B. Swensen, Biochim. Biophys. Acta, 1986, 874, 372. 121. W. H. Flygare, Molecular Structure and Dynamics, Prentice-Hall, Englewood Cliffs, NJ, 1978. 122. R. Sternheimer, Phys. Rev., 1950, 80, 102; 1951, 84, 244; 1952, 86, 316; 1954, 95, 736; 1956, 102, 731. 123. R. Sternheimer, Phys. Rev., 1966, 146, 140. 124. R. M. Sternheimer and H. M. Foley, Phys. Rev., 1953, 92, 1460. 125. H. M. Foley, R. M. Sternheimer and D. Tycko, Phys. Rev., 1954, 93, 734. 126. A. D. Buckingham, Trans. Farad. Soc., 1962, 58, 1277. 127. S. Engstrom, H. Wennerstrom, B. Jonsson and G. Karlstrom, Mol. Phys., 1977, 34, 813. 128. A. C. Legon and J. D. Millen, Chem. Phys. Lett., 1988, 144, 136. 129. A. C. Legon and J. D. Millen, Proc. Roy. SOC.A , 1988, 417, 21. 130. J. Baker, A. D. Buckingham, P. W. Fowler, E. Steiner, P. Lazzeretti and R . Zanasi, J . Chem. SOC.,Farad. Trans. 2, 1989, 85, 901. 131. G. Bacskay and J. E. Gready, J . Chem. Phys., 1988, 88, 2526. 132. G. Bacskay, D. 1. Kerdraon and N. S. Hush, Chem. Phys., 1990, 144, 53. 133. P. W. Fowler, Chem. Phys. Lett., 1989, 156, 494. 134. P. W. Fowler, Chem. Phys., 1990, 143, 447. 135. P. Lazzeretti, E. Rossi, F. Taddei and R. Zanasi, J . Chem. Phys., 1982, 77, 2023. 136. P. Lazzeretti and R. Zanasi, J . Chem. Phys., 1982, 77, 2448. 137. H. Sekino and R. J. Bartlett, J . Chem. Phys., 1986, 85, 3845. 138. A. Laaksonen, J. Kowalewski and V. R. Saunders, Chem. Phys., 1983, 80, 221. 139. A. C. Diz, C. G . Giribet, M. C. Ruiz de Azua and R. H. Contreras, Int. J . Quant. Chem., 1990, 37, 663. 140. R. R. Biekofsky, A. B. Pomilio and R. H. Contreras, J . Molec. Struct. (Theochem.), 1990,210,211. 141. A. V. Afonin, M. V. Sigalov, S. E. Korustova, I. A. Aliev, A. V. Vaschenko and B. A. Tromifov, Magn. Reson. Chem., 1990, 28, 580. 142. H. Satonaka, K . Abe and M. Hirota, Bull. Chem. SOC. Japan, 1987, 60, 953; 1988, 61, 2031. 143. C. A. Parish, J. D. Augspurger and C. E. Dykstra, J . Phys. Chem., 1992, 96, 2069. 144. J. Cizek, J . Chem. Phys., 1966, 45, 4256; Adv. Chem. Phys., 1969, 14, 35. 145. R. J. Bartlett, Ann. Rev. Phys. Chem., 1981, 32, 359. 146. R. J. Bartlett, J. Paldus and C. E. Dykstra, in Advanced Theories and Computational Approaches to the Electronic Structure of Molecules (ed. C. E. Dykstra), p. 127. Reidel, Holland, 1984. 147. J. W. Cooley, Math. Comput., 1961, 15, 363. 148. P. L. Cummins, G . B. Bacskay and N. S. Hush, J . Chem. Phys., 1987, 87, 416. 149. R. D. Amos, Chem. Phys. Lett., 1979, 68, 536. 150. E. Oldfield, K. Guo, J. D. Augspurger and C. E. Dykstra, J . A m . Chem. SOC.,1991, 113, 7537.

NMR Applications to Porous Solids P. J. BARRIE Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ. UK

1. Introduction 2. NMR studies of microporous materials 2.1. Zeolite molecular sieves 2.2. Aluminophosphate-based molecular sieves 2.3. Other microporous materials 3. NMR studies of mesoporous materials 3.1. Silicas, aluminas etc. 3.2. Novel mesoporous materials 3.3. Catalysts 4. 129XeNMR studies of porous materials 4.1. 129XeNMR of zeolites 4.2. 129XeNMR of other porous solids 5. NMR studies of molecular transport in porous solids 5.1. Pulsed field gradient measurements 5.2. Applications of NMR imaging Acknowledgement References

37 38 38 55 62 66 66 72 73 75 75 79 81 82 84

85 85

1. INTRODUCTION

NMR techniques have become increasingly important methods for characterizing materials. In particular, high-resolution solid-state NMR spectroscopy has become a routine tool in the study of many materials as it provides detailed structural information on the local environment and dynamic behaviour of the nucleus under investigation. Porous solids have been extensively studied by this technique due to their great commercial applications, particularly in the area of heterogeneous catalysis. One major NMR application to porous solids is the investigation of the structure of the solid, and to gain information about the solid surface where possible. Another area is investigating the behaviour of adsorbed species. These may be probe molecules which reveal something about the structure of the solid. Alternatively, it may be the dynamics and transformations of the adsorbed ANNUAL REPORTS O N NMR SPECTROSCOPY VOLUME 30 ISBN 0-12-505330-4

Copyright 0 1995 Academic Press Limited AN rights of reproduction in any form reserved

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molecules themselves that is the major topic of interest. As well as conventional solid-state NMR techniques on porous solids, there has been increasing use of field gradients in order to make diffusion measurements and in the area of nuclear magnetic resonance imaging. Porous materials may conveniently be divided into the classification of microporous ( ore diameters 500 A).’ In this review, first microporous systems and then macroporous systems are discussed, even though the division between the categories is to some extent arbitrary. Those materials containing both micropores and mesopores are discussed in the mesoporous section. There are two further sections which are on the application of I2’Xe NMR and on NMR studies of molecular transport within porous solids. Any review on a topic as wide as NMR applications to porous solids is necessarily selective. While this chapter is a long way from being comprehensive, I have tried to give a flavour of the information available from different NMR techniques on a variety of types of porous solids, concentrating mainly on recent methods and discoveries.

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2. NMR STUDIES OF MICROPOROUS MATERIALS

2.1. Zeolite molecular sieves

Zeolites are crystalline aluminosilicates which consist of S O 4 and A104 tetrahedra linked by the sharing of oxygen atoms to form a framework of high internal surface area with regular channels and cavities which permeate the entire volume of the solid. These pores are of molecular dimensions, enabling zeolites to be used as molecular sieves, as they can only adsorb molecules of certain dimensions. Each aluminium atom present in the framework induces a negative charge, requiring a charge-balancing cation to be present in a non-framework position. These cations are relatively mobile and can readily be exchanged by other cations, leading to a number of ion-exchange applications. When the charge-balancing cations become hydrogen ions, zeolites become highly acidic shape-selective heterogeneous catalysts. As such, they have become key catalysts in the oil industry, being used for cracking, alkylation, isomerization and other hydrocarbon transformation reactions. Because of their great commercial applications, zeolites have been extensively studied by NMR spectroscopy, and there have been a number of excellent reviews.2-6 It is appropriate here, however, to consider some of the pertinent points and recent advances in the study of these materials as the various information obtainable by NMR spectroscopy on zeolites is also relevant to the study of other porous solids.

NMR APPLICATIONS TO POROUS SOLIDS

39

2.1.1. "Si N M R of zeolites

The 2'Si environment in zeolites is always tetrahedrally coordinated and so only has a small chemical shift anisotropy. Thus spinning sidebands are normally negligible in magic-angle spinning (MAS) experiments, even at spinning speeds as low as 2 kHz. Following the pioneering work of Lippmaa and c o - ~ o r k e r s 'there ~ ~ was an explosion of interest in performing 29Si MAS NMR experiments on these materials. In general, for a zeolite there may be up to five different 29Si environments present, depending on the number of aluminium atoms connected to them via bridging oxygens. These may be denoted as Si(nA1) where 0 6 n 6 4 , and these give rise to peaks separated from each other by about 5 p p m (depending slightly on the structure, the Si/Al ratio and cations present). The Loewenstein rule,' which states that Al-0-A1 linkages are unfavourable, is believed to apply to all natural and hydrothermally synthesized zeolites and this means that the framework composition (%/A1 ratio) can readily be calculated from the relative intensities of the different Si(nAl) peaks.2,1",11Provided that sufficient time has been left between scans for the spectrum to be quantitative, it may be possible from the relative intensities of the different Si(nA1) peaks to distinguish between possible ordering schemes for the aluminium positions in the framework.12 The determination of the framework Si/AI ratio is a particularly important measurement as it enables various dealumination processes to be monitored. Heating an ammonium-exchanged zeolite in a water vapour atmosphere can result in the removal of some aluminium from the framework which is then replaced by silicon migrating from other parts of the c r y ~ t a 1 . This I ~ process is known as ultrastabilization in the case of zeolite Y (a synthetic version of the mineral faujasite), and gives an increase in the framework Si/AI ratio which results in improved thermal stability and higher acid strength. Most commercial catalysts are in this form. The bulk composition of the zeolite remains unchanged by this process (unless the sample is further treated to remove non-framework aluminium species), and so it is only through the 2ySi MAS NMR spectrum that the framework structural changes can be monitored in detail. 29Si MAS NMR spectroscopy has also revealed that it is possible to reinsert the aluminium species back into the framework by mild hydrothermal treatment with a strong base; this can restore the original Si/Al ratio for the The realuminated sample has, however, a different distribution of Si(nA1) peak intensities, implying a different ordering of aluminium within the framework to the starting material. Other ways of dealuminating the framework without losing significant crystallinity have also been developed including treatment with chelating agents such as EDTA,I7 treatment with SiC14 vapour at high temperatures18 and treatment with aqueous ammonium hexafluorosilicate solution. l9 These other methods have also been studied by 29Si MAS NMR spectroscopy to a greater or lesser extent .20,21

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It is worth noting that the 29Si MAS NMR spectrum provides indirect information about the Si-OH-AI Bronsted acid sites in the hydrogenexchanged form of the zeolite. The different Si(nAl) units for n = 1-4are likely to have bridging hydroxyl units of different acidic strengths. A recent study found four infrared stretches for hydroxyl groups hydrogen bonded to benzene or chlorobenzene in zeolites X and Y, and an attempt was made to correlate the intensities of these with the four types of bridging hydroxyl units anticipated from the 29SiNMR spectrum.22 The situation is more complicated for zeolites with more than one crystallographic site, such as mordenite, offretite and zeolite omega.23In the case of zeolite omega, for example, there are two distinct crystallographic sites in the relative population ratio of 2:l corresponding to sites in the 12-membered rings, and sites that are only in the eight-membered rings. The difference in chemical shift between these sites is such that there is overlap between the Si(nAI)* peaks from site A with the Si(n+l A1)B peaks from the other site. This means that a simple interpretation of the spectrum to obtain the framework SUAI ratio is not possible. Allowance for the existence of the two sites during deconvolution of the spectrum does lead to a satisfactory analy~is.’~In those cases where there is more than one crystallographic site, it may be necessary to dealuminate the sample so that the 29Si NMR spectrum shows only the Si(OA1) peaks. Such a spectrum immediately shows the number of distinct crystallographic sites in the zeolite, and allows their position to be determined which can be an important aid in the analysis of the spectrum of the sample before d e a l ~ m i n a t i o nThe . ~ ~positions of the peaks are, however, shifted slightly by the change in SUAI ratio of the framework. Another method of determining the true position of distinct Si(0AI) peaks without dealuminating the sample was recently demonstrated by Anderson who measured spin-echo Fourier transform (SEFT) spectra of zeolites using different echo times.25 This can give T2-selective spectra in which only those environments which have comparatively long T2 relaxation times appear. In the case of zeolites it is found that, due to the influence of aluminium in the first coordination sphere, Si(nA1) with n>O have significantly shorter T2 relaxation times than Si(0AI) sites. Thus in the case of modernite the spectrum recorded after the spin-echo shows only Si(OA1) environments (see Fig. 1). This method also successfully identifies the Si(0AI) peak positions of the two distinct crystallographic sites in zeolite omega.25 A number of correlations between 29Si chemical shift and structural parameters such as mean Si-0-T bond angle (where T denotes either Si or Al) for both Si(OA1) and Si(nAl) peaks have been suggested.2c30 Perhaps the most reliable of these are the semi-empirical correlations of Engelhardt and Radeglia which show a linear dependence of chemical shift on cosO/(cosO - 1) where 8 is the mean Si-0-T bond angle.26,27 The sensitivity of the 29Si chemical shift to even small changes in local


-80

- 100

41

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6 /PPm Fig. 1. T2-selective "Si MAS NMR spectra of mordenite recorded using a SEFT pulse sequence with increasing echo delays from top to bottom. The delay between each spectrum is 51.2 ms. Note that resolution of the distinct crystallographic sites of the mordenite structure is not possible at this Si/AI ratio. (Reproduced with permission from M. W. Anderson, Magn. Reson. Chem., 1992, 30, 898.).25

geometry means that even the charge-balancing cations present have an influence on the spectrum. This may be useful as, for example, it enables NMR to demonstrate that cation exchange can take place between crystallites even in the solid state.31 Another recent utilization of the influence of cations affecting 29Si chemical shifts is in distinguishing between faujasite (cubic symmetry, denoted FAU) and its hexagonal symmetry analogue (sometimes referred to as Breck's structure six, denoted EMT) which coexist in zeolites ZSM-2 and ZSM-3 for example.32 Figure 2 shows that in the 29Si spectrum of the Na-exchanged forms of zeolite ZSM-2 the Si(nA1) peaks from the two polymorphs completely overlap. However, in the Li-exchanged forms the chemical shifts from the faujasite phase occur at higher frequency by about 3 ppm, while the chemical shifts in the hexagonal analogue are unaltered. This allows the relative amounts of the two phases to be determined, as well as their framework composition^.^^ Another way that cations can influence the 29Si spectrum will take place if the cation is paramagnetic. It has been recently demonstrated that partial ion exchange with Cu2+ in zeolite Y can cause selective broadening of the Si(4Al) peak, thus indicating the site preference for Cu2+ cations.33 The resolution of 29SiMAS NMR spectra of zeolites is mainly impaired by

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I

( I AllF + (2AllE

/

(4AIIF

4

(I AllE $

I

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-100 6 /PPm (a)

I

1

-120

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I

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(b)

Fig. 2. "Si MAS NMR spectra of (a) Na-ZSM-2 and (b) Li-ZSM-2. The peak labels give the number of surrounding aluminium atoms for the faujasite (denoted F) and EMT (denoted E) components. (Reproduced with permission from J. A. Martens et al., J . Phys. Chem., 1993, 97, 5132. 01993, American Chemical S~ciety.)~'

the dispersion of chemical shifts within the sample, which is caused predominantly by the influence of aluminium slightly distorting the structure in its environment. Hence it is found that in highly crystalline dealuminated zeolites extremely narrow 29Sipeaks may be obtained, down to linewidths of 6 Hz in the case of silicalite (the completely siliceous form of zeolite ZSM-5) in which 21 of the 24 crystallographic sites can be resolved.34 Resolution at this level allows subtle changes in structure with temperature and phase changes in the presence of adsorbates to be e ~ a m i n e d .29Si ~ ~ ?spectra ~ ~ of dealuminated zeolites of complicated or unknown structure are particularly useful as they may be correlated with structural parameters and complement crystallographic The ability to observe narrow peaks in the 29Si spectrum of siliceous zeolites also offers the possibility of observing two-dimensional spectra of these materials in order to establish connectivities. The first such studies were performed by Fyfe and co-workers on a 29Si-enriched sample of ZSM-39 which still contained the organic template used in the synthesis.38339 This allowed cross-polarization to be used in order to shorten the delay time between scans which, together with the 29Si enrichment, meant that the applicability of different two-dimensional techniques could be explored in a reasonable time period. Figure 3 shows the two-dimensional COSY spectrum of ZSM-39 at 373 K, which clearly reveals cross-peaks between the Sil and Si2 sites, and between the Si2 and Si3 sites in agreement with the known


43

3 1-124

I

Si2Si3

-112

t4 -108

@ sil si2

-104

-108

-112

-116

-120

-124

-104

6 /PPm Fig. 3. *'Si COSY experiment on ZSM-39 at 373 K. (Reproduced with permission from C. A. Fyfe et a!., J . Am. Chem. SOC., 1989, 111, 7702. 0 1989, American

Chemical S~ciety.)~'

structure. The additional structure in the Si3 cross-peak is real and reflects the absence of a three-fold symmetry axis in the sample studied. However, it is known that COSY experiments depend critically on T; relaxation and the experimental parameters need to be carefully chosen to optimize crosspeaks. Indeed, a COSY experiment at room temperature on ZSM-39 failed to detect the cross-peak between the Sil and Si2 sites as the TZ value of the Sil site at room temperature is relatively short.39 The possible use of double-quantum filtered COSY and spin-diffusion measurements was also explored.39 Fyfe and co-workers then extended their study to more complicated zeolite structures with natural *'Si abundance, investigating siliceous ZSM-12, ZSM-22, ZSM-5, ZSM-11 and zeolite DD3R.4w4 In the case of ZSM-12, an optimized COSY experiment revealed splittings in the cross-peaks in the F2 dimension indicating two-bond J(29Si-29Si) couplings between 9 and 1 6 H ~ . ~ This ' observation is particularly important as it

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P. J . BARRIE

enables two-dimensional INADEQUATE experiments to be performed. These have the great advantage that they remove the strong diagonal peaks from the spectrum and so weak cross-peaks close to the diagonal can be readily identified, but suffer the disadvantage that the technique is rather insensitive and extremely long experiment times are required. Notwithstanding this difficulty, successful INADEQUATE experiments have been performed and these have enabled the complete assignment of the 29Si peaks in the zeolites investigated, including the extremely complicated siliceous ZSM-5 structure in the presence of different adsorbate^.^^ Use of the J-scaled COSY technique has also been suggested as a useful assignment tool and demonstrated for siliceous m ~ r d e n i t e It . ~ can ~ be concluded that two-dimensional 2ySiNMR techniques can be extremely powerful characterization tools, but they depend critically on the sample giving narrow NMR lines. 2.1.2. 27AlNMR of zeolites

27Al MAS NMR spectra of hydrated zeolites normally only show a single peak in the range 50-65 ppm indicating tetrahedral environments, other coordinations being forbidden by the Loewenstein rule. The exact position of the peak depends on the structure and on the magnetic field as 27Al is affected by the second-order quadrupole interaction. In some cases it is possible to resolve more than one tetrahedral peak due to different crystallographic site^,^^,^^ but this is the exception rather than the rule even at very high magnetic fields. 27Al spectra of dehydrated zeolites may give far broader peaks; this is principally due to the fact that the aluminium sites experience higher electric field gradients and hence larger quadrupole coupling parameters. It has recently been shown by using a spin-echo method on a static sample that 27Al sites balanced by sodium cations in dehydrated zeolites Y and ZSM-5 have quadrupole coupling constants of 4.7 and 5.5 MHz respectively, while 27Al sites balanced by acidic hydroxyl groups have quadrupole coupling constants of 12.7 and 16 MHz .~ ' These latter values are so high that MAS techniques at accessible spinning speeds are inappropriate for the study of such environments. They also mean that caution is needed in the interpretation of 27Al spectra of dehydrated zeolites as some broad aluminium components may not be detected (depending on the method used). Hence, unless it is the structure of the dehydrated zeolite itself that is of interest, it is common to ensure that the zeolite is fully hydrated before acquiring the 27Al spectrum in order to avoid this difficulty. As well as tetrahedral aluminium environments in the zeolite framework, 27Al NMR will also detect any non-framework aluminium species. These are often, but not always, in octahedral coordination and so give rise to a well-separated resonance near 0 ppm. The investigation of non-framework aluminium has been a major topic of interest as such species can enhance


45

catalytic activity, probably by providing Lewis acid sites, and such species are present in steam-treated zeolite Y which is used on a large scale commercially. In early solid-state 27Al NMR work on these materials, the relative ratios of the 27Al peaks for tetrahedral and octahedral aluminium were often different from that expected on the basis of elemental analysis and the 29Si results. This difference was caused in part by the experimental conditions used, and in part by Si-OH defect environments complicating the 29Sispectrum. For quantitative work on half-integral quadrupolar nuclei in solids it is necessary to use very short pulses ( < d l 2 flip-angle for a spin I = 5/2 nucleus) and strong r.f. pulse^.^^'^ It is also advantageous to use as high a magnetic field as possible due to the second-order quadrupole interaction, and very fast magic-angle spinning to avoid overlap with spinning sidebands. 27Al spectra of steamed zeolite Y samples have been the topic of considerable debate, as in a number of spectra a peak at about 30ppm has been detected, as well as peaks at about 60ppm (framework tetrahedral) and 0 pprn (non-framework octahedral). This additional peak has been variously assigned to a tetrahedral environment with a high quadrupole coupling constant ,’* or to a five-fold coordinated en~ironment.’~The situation has been extensively studied by a variety of methods including two-dimensional quadrupole nutation” and ‘H-27Al cross-p~larization.’~~~’ The latter technique clearly showed that the peaks at 30 and Oppm were enhanced relative to that at 60ppm7 suggesting that the 30ppm species originates from a distinct five-coordinate environment. However, it is dangerous to draw firm conclusions from 27Al CP/MAS spectra of this nature as the different crystallite orientations present giving rise to the quadrupolar powder pattern may cross-polarize at differing efficiencies and so give rise to unexpected line shape^.'^ Much of the argument has finally been resolved in a recent double-rotation (DOR) study by Ray and S a m o ~ o n . ’They ~ investigated three samples of zeolite Y thermally treated in the presence of water vapour, two of which had been subjected to a single thermal dealumination removing about 25% of framework aluminium (samples LZ-Y72 and LZ-Y82), and one of which had been subjected to a second thermal treatment to remove about 65% of the framework aluminium (sample LZ-Y20). The 27Al MAS NMR spectra of these samples in both 14.1 T and 9.4 T magnetic fields were fairly similar, though the peak at about 30ppm was stronger in the sample that had experienced two heat treatments. The 27Al DOR spectra, however, gave different results for the samples, as shown in Fig. 4. The two samples which had a single thermal treatment gave tetrahedral peaks at 59 pprn and 47 ppm, which indicates in this case the peak at 30ppm in the MAS spectrum results primarily from distorted tetrahedrally coordinated aluminium (with a quadrupole coupling constant of the order of 6 MHz). The peak at 20-25 ppm is believed to be due principally to a spinning sideband. On the other hand, the sample which

46

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I.

I

I

00

1

I

I

I

I

0

40

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6 /PPm Fig. 4. ”A1 DOR NMR spectra of steam-treated zeolites: (a) LZ-Y20 (after two heat treatments), (b) LZ-Y82 and ( c ) LZ-Y72 (both after a single heat treatment). (Reproduced with permission from G . J . Ray and A. Samoson, Zeolites, 1993, 13, 410.)”

had been given two thermal treatments did not give narrower peaks on DOR, indicating that the linewidths were caused mainly by chemical shift dispersion rather than second-order quadrupole effects. Thus in this sample, the peak at 30 ppm arises principally from five-coordinate aluminium. The fact that this peak occurs at this position independent of magnetic field together with the results of the DOR experiment imply that this site has a fairly low quadrupole coupling constant. It is now known that this is indeed possible for certain five-coordinate aluminium g e ~ m e t r i e s .Hence ~~ it appears that the nature of the peak at 30 ppm depends on the sample’s exact thermal history, and it may have contributions from both four-coordinate and five-coordinate aluminium species. There has been another recent DOR


47

study on ultrastable zeolite Y, but the details of the sample preparation were not given.s9 This latter study found that most of the central peak was due to distorted tetrahedral aluminium, and pointed out that aluminium sites in zeolites can experience a broad distribution of electric field gradients and quadrupole interactions. It should be mentioned that while D O R removes broadening due to the second-order quadrupole interaction, much of the 27Al linewidth in the spectra of hydrated zeolites recorded at high magnetic fields arises from the effects of chemical shift dispersion, and so D O R has only limited applicability to the study of these materials.

2.1.3. N M R of Brgnsted acid sites One particularly useful experiment when studying zeolites is to look at the Bronsted acid sites directly using 'H NMR spectroscopy.6@62 This is, however, not quite as straightforward as it may first appear, as it is necessary to perform the experiment on dehydrated samples otherwise water would mask the signals of interest. For high-resolution work the dehydrated sample may be sealed in a glass ampoule designed to fit snugly inside the rotor for MAS studies, or alternatively packed into a specially designed air-tight rotor inside a glove-box. Fortunately for most zeolites the distance between 'H environments is sufficiently large that MAS alone is sufficient to give high-resolution spectra without using multiple-pulse line narrowing methods which can be difficult to implement successfully. The Si-OH-A1 B r ~ n s t e d acid sites in zeolites may give resonances between about 3.8 and 5.2ppm depending on acidic strength (affected by the Si/AI ratio) and the detailed structure. In zeolite Y, for example, two Brmsted sites are observed: one at about 4 p p m (peak B) due to bridging OH groups pointing towards the large supercage, and the second at about Sppm (peak C) arising from bridging OH groups pointing towards the smaller sodalite cages.63 However, 'H spectra of zeolites are rarely that simple as there will also be peaks due to non-acidic silanol hydroxyl groups at the surface and at crystal defects (1.3-2.3 ppm), from any Al-OH groups attached to non-framework aluminium species (2.5-3.5 ppm) and from any residual NH4+ cations (6.5-7.0 ppm). Attempts have been made to correlate the chemical shift of the hydroxyl groups with their infrared stretching frequencies.a The question of the inherent resolution of 'H MAS studies of zeolites has been addressed by B r u n r ~ e rwho ~ ~ showed that in aluminiumrich zeolites very fast spinning and high magnetic fields are advantageous as they remove the effects of residual 'H dipolar broadening. In medium strength magnetic fields at moderate spinning speeds partial deuteration of the sample can significantly enhance resolution (see Fig. 5).65 It is not always the case that high-resolution 'H spectra are desired as

48

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20

0

1 0

-10

6 /PPm Fig. 5. 'H MAS NMR spectra recorded at 300 MHz with 3 kHz MAS: (a) zeolite H-Y (Si/Al = 2.6), and (b) partially deuterated sample. (Reproduced with permission from E. Brunner, J . Chem. Soc., Faraday Trans., 1993, 89, 165.)65

static 'H spectra can give information about distances between nuclei. Thus the second moment of the static 'H spectrum has been used to calculate that the mean distance between a framework aluminium atom and the acidic bridging hydroxyl hydrogen atom is 2.38 ? 0.04 A in zeolite H-Y and 2.48 ? 0.04 &. in zeolite H-ZSM-5.66 Spinning sideband intensities in the 'H MAS experiment have also been analysed giving an Al-H distance of 2.5 A in zeolite H-ZSM-5.67 More recently, the spin-echo double resonance (SEDOR) technique has been applied to measure 27Al-iH dipolar coupling directly, and this gives an A1-H distance of 2.43 f 0.03 Thus it is clear that NMR can be used to obtain detailed information about the geometry of the Brmsted acid sites, which may be compared with the results of ab initia quantum chemical calculation^.^^ Wideline 'H NMR has also been used as the basis of a proposed acidity


49

scale by measuring spectra at very low temperatures (4K) on samples containing the same number of water molecules as Bronsted acid sites.70 The resulting dipolar-coupled spectra may be fitted to include contributions from H 3 0 + ions, H 2 0 - H O hydrogen-bonded groups, O H groups of the solid and unbonded H 2 0 molecules. In zeolite H-Y, for example, it is found that with increasing water content the number of H 3 0 + ions and H 2 0 - H O groups increases at the expense of acidic OH groups in the solid not bonded to water. Above a concentration of one water molecule per Brmsted acid site there is no change in the concentrations except that a contribution from non-bonded H 2 0 is now ~ b s e r v e d . ~These ' results provide a basis for measuring acidities of Brmsted sites in solids as high proportions of H30+ groups will be detected in highly acidic solids, predominantly H 2 0 - H O groups will be detected for mildly acidic solids, while non-acidic solids will not give rise to either group. 'H MAS has also been used profitably to look at deuterated Brmsted acid sites in zeolites,48 and the quadrupole coupling constant of the Si-OD-A1 sites increases with framework aluminium content of the zeolite from 208 kHz (zeolite H-ZSM-5) to 236 kHz (zeolites H-X and H-Y). Probe molecules that are basic may also be used to study acid sites in zeolites. Bases such as ammonia, trimethylamine, pyridine, acetonitrile and trimethylphosphine may bond to Bransted or Lewis acid sites, and these may be studied by 'H, 13C, "N or 31P NMR as appropriate. 'H NMR chemical shifts of ammonia, water and methanol molecules interacting with Bronsted acid sites have been calculated using ab initio calculations.71 Experimental 'H MAS NMR spectra of methanol adsorbed inside zeolites show that the O H resonance position is highly influenced by the formation of methoxonium ions and by the extent of hydrogen bonding within the pore system.72 For instance, the methanol O H resonance comes at 9.4ppm in H-ZSM-5, 5.1 ppm in H-Y, 4.7 ppm in Na-Y and 3.6 pprn in Na-ZSM-5. 'H MAS NMR has also been used to observe hydrogen bonding as a function of temperature for acetylene, ethylene, carbon monoxide and benzene in zeolite H-ZSM-5.73 I5N NMR spectroscopy of "N-enriched ammonia adsorbed in zeolite H-Y (recorded with or without cross-polarization) can give peaks due to NHZ cation^.^^,^' The number of "N resonances observed depends on sample loading as there can also be peaks from NH3 hydrogen-bonded to NH: cation^.^' 'H MAS NMR has also recently been used to study ammonia adsorbed inside zeolite Y with different loadings and different cations.76 There are at least two ammonium species which show fast hydrogen exchange with B r ~ n s t e dhydrogen atoms; these probably arise from the difference between the acid sites in the supercages and in the sodalite cages. A peak due to NHZ without exchange may also be detected at 8.1 ppm in some cases. The results of 15N studies of trimethylamine adsorbed within zeolite H-Y clearly show the formation of [(CH3)3N-H]+.75 The spectra

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P. J. BARRIE

only show a single peak, presumably because hydrogen-bonding effects are negligible with this base. The larger size of this base should preclude entry into the smaller sodalite cages. The 15N linewidth observed depended on sample history.75 Studies of pyridine inside zeolites using I5N NMR can in principle clearly distinguish between physisorbed species, Lewis acid sites and pyridinium ions (bonded to B r ~ n s t e dacid sites).77 'H NMR spectra of pyridine have also been used to distinguish between accessible and non-accessible Bransted acid sites in zeolites.63 The use of trimethylphosphine as a sensitive probe that can be studied by 31PNMR has been similarly explored, and in the absence of proton decoupling, J-coupling between 31Pand 'H can sometimes be observed for [(CH3)3P-H]+ cations (Fig. 6).78,79Species due to trimethylphosphine bonded to different Lewis acid sites arising from non-framework aluminium in dealuminated zeolites can also readily be detected.797s0In the event that oxygen is not rigorously excluded from the sample, then there may also be 31P peaks from phosphine oxide environments.80 While acid sites in zeolites have received much attention, some reactions may occur through basic sites, and there has been a recent suggestion that nitromethane may be a suitable probe molecule for investigating these." 2.1.4. N M R studies of other nuclei in zeolites The other framework element in zeolites is oxygen, and this too can be studied by solid-state NMR if a degree of I7O isotopic enrichment is used. The lineshapes are dominated by the second-order quadrupole interaction. Due to their difference in quadrupole coupling constants, Si-0- A1 environments may be distinguished from Si-0-Si environment^.^^,^^ Values of the quadrupole coupling constant may either be obtained from static spectra (which has the disadvantage that there are additional contributions to the lineshape from chemical shift anisotropy), or from MAS spectra (which has the disadvantage that the quadrupolar features are less distinct). Si-O- Al environments typically have NMR parameters of e2qQ/h = 3.1-4.3 MHz, 17 = 0.2 and ais0 of 31-45ppm, while Si-0-Si environments have 2qQl h = 4.6-5.8MHz7 17 = 0.1 and ais0of 4 4 - 5 7 ~ p m . ~ ~ , * ~ A number of other elements can be isomorphously substituted into zeolite frameworks to a greater or lesser extent. Gallium can be incorporated into the framework of a number of zeolite structures, and it affects 29Sispectra in a similar fashion to aluminium.84 The @Ga and 71Ga isotopes can also be studied directly by NMR. Due to the differences in Larmor frequencies and quadrupole moment of these isotopes (both of which are spin Z = 3/2), they experience different second-order quadrupole interactions, and so measurement of both the 69Ga and 71Ga isotopes enables an estimate of the quadrupole coupling magnitude to be ~ b t a i n e d . ' ~In the case of Gasubstituted H-ZSM-5 it is found that the quadrupole coupling constant at


240

160

00

0

-80

-160

51

-240

6 /PPm Fig. 6. 3'P MAS NMR spectra of PMe3 adsorbed on zeolite H-Y: (a) with 'H decoupling showing peaks at -67 and -2 ppm due to physisorbed and chemisorbed PMe3 respectively, and (b) without 'H decoupling, showing 3'P-1H J-coupling for the

chemisorbed species, and spinning sidebands resulting from dipolar coupling to protons. (Reproduced with permission from W. P. Rothwell et al., J . Am. Chem. SOC.,1984, 106, 2452. 01984, American Chemical Society.)78 the G a sites is rather larger than for analogous aluminium environments (69Ga 3.0MHz7 7'Ga 1.9MHz, 27Al O . ~ M H Z ) . 'The ~ loss of tetrahedral gallium upon thermal treatment in a process akin to dealumination has also been explored in Another element that can have a profound effect on catalytic behaviour when in the framework is titanium. The 29Si spectrum of TS-1 (the titanium-containing analogue of ZSM-5) shows broad peaks due to Si(4Si) and Si(3Si,lTi) environments, and changes with temperature." Modification of zeolite H-ZSM-5 by impregnation with phosphoric acid is one way of altering catalytic activity and selectivity, and this reaction has also been studied by NMR.89-91 It appears that the acid initially causes partial dealumination resulting in a reduction of Brmsted acid sites and the formation of amorphous aluminium phosphate within the pores. However, subsequent thermal treatment of the modified zeolite can give a lower degree of dealumination than would have been the case for the unmodified zeolite. Hence the treatment can affect catalytic activity, selectivity and diffusivity. There is no evidence for phosphorus incorporation into the framework. There have been comparatively few studies of cation environments in

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zeolites directly by NMR. This is because most of the cations of interest are quadrupolar. In the case of sodium cations, 23Na NMR spectra show a fairly narrow average peak for hydrated zeolites, while in the dehydrated form only a broad peak is observed due to a large increase in quadrupole coupling constant. There have recently been several 23Na studies of dehydrated zeolite Na-Y using D O R which gives a significant improvement in resolution. The first of these observed three peaks, which were assigned to Na in sites I , I' and II.92-94 However, another study only detected two 23Na e n ~ i r o n m e n t s while , ~ ~ a more recent study concluded that there were four distinct 23Na sites on the basis of simulation of the spinning sideband^.'^ These differences in assignment may be due in part to the magnetic field dependence of the second-order quadrupole interaction; it is also possible that traces of residual water affect the results. 23Na D OR spectra were also obtained on loading dehydrated zeolite Na-Y with water and t r i m e t h y l p h ~ s p h i n eThese . ~ ~ cause significant changes in sodium environment due to coordination to the cation. The interaction of the sodium cations with Mo(CO)~has also been explored.93 This is of interest as M o ( C O ) ~can be converted to M o o 3 inside the zeolite. In slight contrast to the 23Na D O R results,95 Mo TI measurements on M o ( C O ) ~inside zeolite Na-Y indicate that Mo(CO)~experiences significant rotational freedom within the supercages, with a correlation time at room temperature three orders of magnitude higher than in solution.97 The motion of adsorbates within zeolites has been the subject of a large number of studies. lZ9Xe NMR and pulsed field gradient (PFG) NMR studies to measure diffusion coefficients of adsorbates are discussed in later sections of this review. Static *H NMR spectra as a function of temperature have proved to be particularly useful in characterizing molecular reorientations of deuterated organic molecules inside zeolite^.^^^^ For instance, *H NMR shows the existence of a binding site for C6D6 in zeolite Na-Y, presumed to be sodium cations on site SII, while isotropic reorientation of benzene occurs even at temperatures as low as 155 K within a siliceous form of zeolite Y containing no cations."" A recent advance in the study of organic molecules within zeolite molecular sieves is the use of multiplequantum NMR to measure the apparent spin network size as a function of excitation time. This has been applied to 'H spins in hexamethylbenzene and benzene inside zeolite Na-Y. In principle the results correspond to the number of 'H spins present inside a zeolite cavity, thus giving the average number of molecules within each cavity. 1023103

2.1.5. In-situ NMR studies of catalysis by zeolites There has recently been an upsurge in using NMR to study catalytic reactions taking place inside the zeolite. The general approach is to use I3C-enriched reactants, and to seal the sample inside an air-tight container


53

such as a glass ampoule.lm Spectra may then easily be measured at room temperature after heating the sample at a particular temperature for a known time ,105,106 or alternatively acquired truly in situ by heating the sample in the probe. lo' A whole range of hydrocarbon transformations inside zeolites have now been observed by these methods, and the 13C NMR spectra have shed new light on reaction intermediates, reaction mechanisms and aspects of shape selectivity. There has been an elegant series of papers by Haw and co-workers who have developed designs for preparing catalyst-adsorbate samples suitable for in situ MAS work while still allowing subsequent adsorption of additional reagents to be made. 108~109The same group have also successfully adapted a probe to allow temperatures as high as 673 K to be reached so that zeolite catalysts can be studied at the same temperature at which they are employed commercially. 'lo The conversion of methanol to gasoline in a single step (MTG reaction) using zeolite H-ZSM-5 has been studied over many years, but the mechanism, particularly with regard to the formation of the first C-C bond, remains a subject of debate. This reaction is one of the most successful routes to synthetic fuels, and now produces one third of New Zealand's gasoline supply. 13C relaxation time measurements of methanol adsorbed within H-ZSM-5 have shown that there may be up to three different species present at room temperature depending on SUA1 ratio. The variable temperature 13C NMR studies have shown that methanol is first dehydrated to dimethyl ether at relatively low temperatures. After heating to temperatures of 573 K and higher a whole range of aliphatic and eventually aromatic products form within the zeolite (see Fig. 7). The distribution of products inside the zeolite is different from that expected on thermodynamic grounds, indicating shape selectivity at the active site, and differs from the distribution of products that can escape from the zeolite due to product selectivity. 105~106High-temperature in situ studies have shown that ethylene is the first olefin produced in the reaction and that water from the dehydration reactions can substantially modify the observed acidity. '12 The mobility of the adsorbed species means that conventional two-dimensional methods may be used to help with peak assignment^.'^^^'^^ In some cases carbon monoxide may be observed before the transformation to higher hydrocarbons. This was initially suggested to be a reaction intermediate of potential ~ i g n i f i c a n c e , ~but ~ ~ ~this ' ~ ~has subsequently been questioned following results showing that addition of 13C-enriched carbon monoxide and natural abundance methanol to zeolite H-ZSM-5 did not result in the formation of any hydrocarbons containing significant amounts of 13C.'15 The most popular proposed mechanism for C-C bond formation involves the formation of a trimethyloxonium ion. This proposal suffers the drawback that this ion has not been detected in 13C NMR results on methanol conversion. It has, however, been detected at room temperature in the expected position of 80 ppm for H-ZSM-5 samples loaded with dimethyl

"'

54

P. J. BARRIE

z 5 min at 523 K

45min at 5 2 3 K

I

6 0 min at 523 K

I

I 75 min at 523 K

250

200

150

100 8 /PPm

50

0

-50

Fig. 7. In situ I3C MAS NMR spectra showing the conversion of "C-methanol to hydrocarbons on ZSM-5 recorded at the temperatures indicated. Expansions of the aromatic region are also shown. (Reproduced with permission from E. J. Munson ef al., J. Phys. Chem., 1992, 96, 7740. @ 1992, America1 Chemical Society.)"'

ether (a known intermediate in the MTG process).'16 Thus the exact mechanistic role for trimethyloxonium ions in this reaction is still uncertain. There have recently been further studies of trimethyloxonium, trimethylsulphonium and trimethylselenonium ions in zeolites using in situ I3C NMR."' The conversion of olefins inside zeolites has also been studied, and reaction intermediates have been detected at low temperatures (down to 143K)."s121 It is beyond the scope of this review to discuss all the other acid-catalysed reactions which have been studied by in situ I3C


55

~~~,107,110,122-125 though the power of the technique has hopefully been demonstrated. One other catalytic reaction studied by NMR is worthy of mention as it proceeds essentially via a basic reaction mechanism, unlike most transformations in zeolites. It is also of interest because this reaction was discovered by NMR. 13C NMR shows that methyl iodide forms framework-bound methoxy species within alkali-metal exchanged zeolites such as Cs-X,126,127and these react to form ethylene and other hydrocarbons at temperatures as low as 500 K.'26

2.2. Aluminophosphate-based molecular sieves Since 1982, several new families of molecular sieve based on aluminophosphate framework structures have been synthesized, and these exhibit many properties similar to those of aluminosilicate zeolites. The first such family, known as the AlP04 molecular sieves, is composed of alternating A104 and PO4 tetrahedra. As they have neutral frameworks, the A l P 0 4 family possess no exchangeable cations, but are of potential application as molecular sieves and hosts for guest materials. 128,129 Some of these materials have the framework topologies of known zeolites, but many have novel crystal structures. 13" Incorporation of a reactive source of silicon into an aluminophosphatebased synthesis mixture can result in the formation of framework silicoaluminophosphates (denoted SAPO), while addition of metals can give metalloaluminophosphates (MeAPOs). The metals most commonly used are magnesium, manganese, iron, cobalt and zinc, though others such as vanadium, chromium, nickel and tin have also been claimed to be incorporated into the framework structures. Some of these new materials have the framework topologies of known zeolites or AlP04 structures, while many have novel structures. 1 3 1 ~ 1 3 2The SAPO and MeAPO molecular sieves resemble zeolites in that, as well as being molecular sieves, they have negatively charged frameworks balanced by cations located within the channels. Thus they have potential uses as ion-exchangers and catalysts. A number of other elements have also been claimed to substitute into the framework of aluminophosphate-based molecular sieves, including lithium, beryllium, boron, titanium, gallium, germanium and arsenic, giving ElAPO materials. It is also possible to accommodate more than one different heteroatom into the framework, and to have both metals and silicon simultaneously in the framework, giving rise to MeAPSO and EIAPSO species. 132 Once again, this leads to several new structural types being formed. This wide variety of novel molecular sieves (over 50 structure types) and their compositional variants offer a large number of parameters to tailor to specific adsorption or catalytic requirements. One particular feature of the

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structure of many of these materials, in contrast to zeolites, is the capability of framework aluminium to bind to one or two extra-framework oxygen species such as -OH or -OH2, and thus have a higher coordination number than four. 133 Prompted by the considerable information gleaned from solid-state NMR studies of zeolites, there have been a large number of NMR investigations into the structure and the ordering of the framework elements in these materials and these have been reviewed.134

2.2.1. Studies of A1PO4 molecular sieves Both 27Al and 31P are sensitive nuclei, so NMR spectra can easily be obtained. 31P chemical shifts for P(OAl)4 environments in AlP04 materials are normally between -23 and - 3 5 ~ p m . lA~ ~correlation between mean P-0-A1 bond angle and 31P chemical shift has been suggested for dense AIP04 phases,135and seems to work respectably in those cases where the 31P is bonded via oxygens to four tetrahedrally coordinated aluminiums. However, there may be significant deviations to higher frequency in those AIP04 materials containing higher coordinated aluminium, and 31P peaks as high frequency as -5.7ppm have been r e ~ 0 r t e d . lGenerally ~~ the 31P spectrum is capable of revealing the number of crystallographic sites, though there may be considerable overlap between the different sites. For example in hydrated AlPO4-ll four of the five sites give rise to a composite peak at -29.6 ppm, while the fifth gives a resolved peak at -23.4 The use of 31P MAS NMR to reveal structural information is well illustrated by the example of VPI-5. This aluminophosphate has attracted great interest since its discovery, as its structure consists of channels circumscribed by 18-membered rings, which are larger than those found in any known aluminosilicate zeolite. 137 Early structural refinements suggested that there were two crystallographic sites for both phosphorus and aluminium in the population ratio of 2:1,138,139and two 31P NMR peaks are indeed observed in this intensity ratio for dehydrated VPI-5.l4' However, for hydrated VPI-5, the 31P MAS NMR spectrum clearly shows three peaks of equal intensity, indicating that in this form there are three distinct sites.14' Following this NMR study and others, there was an improved crystallographic refinement of hydrated VPI-5 which successfully took into account the structure of the water within the channels in order to remove the di~crepancy.'~~ It is interesting to observe that it is possible to transform hydrated VPI-5 to the high-temperature phase even in the presence of water, and this has been elegantly demonstrated by variable temperature NMR. Heating the sample in a sealed rotor so that water vapour cannot escape shows that the water motion within the channels at temperatures above 80°C is such that the high-temperature phase is adopted, and that this transition is completely reversible upon cooling (Fig. 8).142,143


57

A

fl70"C

0,

.-0C

(L 0

V

120°C

80°C

(c)

Q,

c .+

60°C

(b)

0

-20

0

b)

I

-40

6 /PPm Fig. 8. Variable temperature 31PMAS NMR spectra of hydrated VPI-5 in a closed rotor system. (Reproduced with permission from L. Maistriau et al., A p p l . Catal. A : General, 1992, 81, 67.)'43

The fact that framework aluminium in A1P04-based materials can exist in four-, five- and six-fold coordination means that 27AlMAS NMR becomes a particularly useful characterization tool, particularly at high magnetic fields when the complications arising from the second-order quadrupole interaction are reduced. Thus in the case of hydrated VPI-5, 27Al MAS NMR shows that one-third of the aluminium is in octahedral coordination due to additional bonding to two water molecules. The two distinct tetrahedral sites have very similar isotropic chemical shifts, but may be resolved by DOR at magnetic field strengths of 9.4 T or lower as they have different quadrupole coupling parameters. 144~145

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The assignment of the three 27Al and 31P NMR peaks in VPI-5 to specific crystallographic sites is an important question. However, there are conflicting assignments in the literature based on considerations such as the spectral changes that occur with temperature and mean P - 0 - A1 bond angles. More direct information on spectral assignment may be obtained by more sophisticated NMR techniques, and these have been demonstrated on VPI-5 as a model system. Coherence transfer between spin Z = 1/2 and quadrupolar nuclei is possible and both 27Al+31P and 31P+27A1 crosspolarization (CP) spectra have been achieved on VPI-5.'46 Recently it has been demonstrated that improved coherence transfer between these nuclei in VPI-5 occurs if the CP process takes place while the sample is spinning parallel to the magnetic field (and then switched to the magic-angle during d e t e ~ t i o n ) . 'This ~ ~ is due to the significant reduction in TI, relaxation time of quadrupolar nuclei under MAS condition^.'^^ Experiments using pulse sequences based on rotational-echo double-resonance (REDOR) and transferred-echo double-resonance (TEDOR) methods have also been demonstrated on VPI-5; these pulse sequences restore the heteronuclear dipolar coupling between the nuclei under MAS conditions. 14s151 Thus two-dimensional heteronuclear correlation experiments can be performed as an aid to spectral assignment. A two-dimensional 27Al-31P correlation spectrum on VPI-5 is shown in Fig. 9. All three P sites give correlation peaks indicating proximity to both tetrahedral and octahedral aluminium, but the 31P site at -33.6ppm gives a noticeably stronger correlation to the octahedral aluminium site indicating unambiguously that it arises from the P1 site sharing four-membered rings (which is linked via oxygens to two octahedral aluminiums rather than just one). Despite the use of these methods, a full assignment of the 27Al and 31P NMR peaks in VPI-5 to specific peaks still relies on correlations involving bond angles, 27Al quadrupole coupling parameters or cluster model calculations of the interaction of water within VPI-5, and there is still disagreement in the literature.145*152,153 NMR studies have also been performed on molecules within the channels of VPI-5. The small amount of organic material retained during synthesis gives a high-resolution 'H MAS NMR spectrum when acquired using a spin-locking pulse sequence (which has the effect of suppressing the 'H signal from water).'54 13C NMR spectroscopy has been used to show that buckminsterfullerene, C60, may be accommodated intact within the The dynamics of adsorbed molecules such as D 2 0 and ND3 within the channels of VPI-5 have been studied by variable temperature static 2H NMR spectroscopy. 157~158 As the A l P 0 4 molecular sieves are highly crystalline and can have a range of different sites, they have proved ideal candidates to demonstrate the power of the D O R technique to remove second-order quadrupolar linebroadening. As well as the studies on VPI-5,144,145there have been D O R


C'

I

59

I

!

Q

0-

%

-

(0

$ N

w

501

1 0

0

-2 0

-40

-60

31P6/ppm Fig. 9. Solid-state 2D heteronuclear 27Al-31Pcorrelation spectrum of hydrated VPI-5. (Reproduced with permission from E. R. H. van Eck and W. S. Veeman, J . Am. Chem. SOC., 1993, 115, 1168. 01993, American Chemical S~ciety.)"~

studies on AIPO4-11,136,159 AlP04-8 and -14,16' AlP04-18,'61*'62 and AIP0421 and -25.'63 Even though D O R removes the anisotropic part of the second-order quadrupole interaction, there still is a field-dependent shift of the resonance peaks so that even in D O R studies it is helpful to record the The "A1 DOR spectrum at more than one magnetic field spectra have been particularly useful in probing the structural changes that occur on adsorption of polar species into the molecular sieve. In the case of A1P04-18 for instance, a material which is isostructural with the natural zeolite chabazite, there are major changes in structure upon adsorption of methanol, ammonia and water (see Fig. The as-prepared sample contains two tetrahedral and one five-coordinate aluminium site in agreement with the crystallographic structure s01ution.l~~ There is a structural change upon calcination, and then further changes that are reversible upon adsorption of polar species. In the case of saturation with methanol, the D O R spectrum (Fig. 10) shows that there are now four sites present in the ratio 1:2:1:2 with the last being a five-coordinate species. It is interesting to note that the formation of five-coordinate aluminium in this material requires the presence of more than one methanol molecule per cavity.'64 In

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t

80

I

I

I

40

I

0

I

1

I

-40

/PPm Fig. 10. 27Al DOR NMR spectra of A1P04-18: (a) as prepared, (b) methanolsaturated, (c) ammonia-saturated and (d) water-saturated. Dots indicate spinning sidebands. (Reproduced with permission from J. Janchen et al., J . Phys. Chem., 1993, 97, 12 042. 01993, American Chemical Society.)161

contrast, loading with ammonia or water produces substantial amounts of octahedral aluminium instead of five-coordinate aluminium, and little change in the resonance positions of the tetrahedral aluminium sites relative to the as-prepared sample. 164 These results nicely show the use of 27AlDOR NMR for probing changes in structure in the presence of adsorbates. Two-dimensional 27Alquadrupole nutation has also been applied with some success to the study of A1PO4 materials, as has 27Al CP/MAS.16" However, a certain degree of caution is needed in the interpretation of spectra recorded using these techniques as the orientation dependence of the second-order quadrupole interaction can produce unexpected results.56


61

The 1 7 0 environment in some AlP04 molecular sieves has also been observed, and shown to have a rather larger quadrupole coupling than in zeolites or silicates.165

2.2.2. Studies of SAPO molecular sieves The SAPO molecular sieves are frequently considered to be aluminophosphate frameworks with some isomorphous substitution by silicon. The location of the silicon has been a major topic of interest, and three possible mechanisms for silicon substitution can be envisaged:

(1) substitution of silicon into aluminium sites; (2) substitution of silicon into phosphorus sites; (3) simultaneous substitution of two silicon atoms for an aluminiumphosphorus pair. Chemical analyses invariably show that the phosphorus content is less than the aluminium content so that the framework has a net negative charge. This indicates that mechanism 2 must dominate over mechanism 1. However, the amount of aluminium present is often less than that expected if mechanism 2 was occurring alone, so it appears that some painvise substitution (mechanism 3) can also occur. 31P MAS NMR spectra of SAPO materials are normally identical to that of the analogous AlP04 framework, indicating that all phosphorus sites are in P(OAl)4 environments.lM Hence there are no P-0-Si bonds present. 27Alspectra are more complex, and can have broad underlying peaks as well as narrower peaks due to tetrahedral and octahedral aluminium. 29Si MAS NMR spectra tend to be of low quality due to the lower sensitivity of this nucleus and the relatively small amounts of 29Si present. In a number of cases it is clear that the idealized silicon for phosphorus mechanism is obeyed and a single peak for Si(OA1)4environments is while in a number of others there are additional 29Si peaks due to more siliceous environment^.'^^^^^^'^^ These have been suggested to arise from “silica islands” within inhomogeneous crystallites indicating that some pairwise substitution (mechanism 3) is also occurring. 174~175,178However, there is also the complication that small amounts of amorphous silica coating the crystallites will also give rise to 29Sipeaks in this region and the elemental analyses observed. 182,183 As in the case of AlP04 molecular sieves, there may be modifications to SAPO structures in the presence of adsorbates such as water. For instance, water can cause the complete collapse of the structure of SAPO-37. In the case of SAPO-34, on the other hand, 29SiNMR has shown that water causes the opening of Si-0-A1 bonds giving Si-OH and A1-OH species, but this process is reversible on dehydration. lS4

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In an analogous fashion to work on zeolites, 'H MAS NMR spectroscopy has been used to probe Brmsted acid sites d i r e ~ t l y , ~and ~ ~ hydrocarbon ,'~~ transformations within the molecular sieve have been studied by 13C ~~~.169,185 2.2.3. Studies of other AIP04-based molecular sieves Solid-state NMR spectroscopy has provided some of the clearest evidence for the incorporation of metals into the frameworks of MeAPO species. In the case of a sample of MAPO-20 (the magnesium aluminophosphate analogue of sodalite) well-resolved 31P peaks corresponding to P(OA1)3(OMg) and P(OAl),(OMg), are observed for a sample containing 15% magnesium on the tetrahedral sites.'86 Only a single 27Al NMR peak is observed. This shows that the magnesium is present exclusively on the aluminium sites and allows the framework composition to be calculated from the relative areas. The areas also enable alternative ordering schemes for the framework elements to be distinguished. lS6 Other magnesiumcontaining MeAPO species have also been studied by NMR.lS7 However, a degree of caution is needed in using 31P spectra to gain information on the extent of metal substitution, as many of the MeAPO structures have more than one crystallographic site and so give rise to multiple peaks even in the absence of incorporated metal. In those cases where the metal is paramagnetic, the 27Al and 31P spectra show large anisotropies, manifested in an array of spinning sidebands, due to the paramagnetic interaction between the unpaired electrons on the metal and the 27Al and 31P nuclei in the vicinity. 188~189It is possible to simulate the spinning sideband patterns to obtain the paramagnetic coupling, and a calculation on MnAPO-5 suggested that manganese was present in both framework and non-framework positions. 190 2.3. Other microporous materials

2.3.1. Microporous frameworks There are a number of other inorganic microporous framework solids with novel compositions that have recently been synthesized; these include gall oars en ate^'^^ and gall opho~pha t e s,'~ '-'~alurnin~arsenates,'~~~~~ ~ beryllophosphates.2m There are also a range of titanosilicates which have structures related to zeolite^.^^^-^^^ One gallophosphate of significant interest is known as cloverite; its structure consists of large supercages with a body diagonal length of 29-30 A accessed through cloverleaf-shaped 20-membered ring windows (larger than any known zeolite or AIP04).193The large supercage may be of particular


63

use as a host for preparing nanosized particles of potential application in materials science. Several characterization studies of cloverite have now appeared which include the use of NMR. 194,204207 The 31PNMR results are in broad agreement, and indicate a range of tetrahedral phosphorus sites including P-OH environments in line with the crystal structure. 193 However, there are differing interpretations of the 71Ga NMR results. Five gallium sites are expected, including a Ga-OH environment. 'lGa MAS NMR spectra appear to give peaks due to two differently coordinated gallium environments. 194,205 However, static 71Ga spectra recorded on cloverite at two magnetic fields indicate that the second-order quadrupole broadening is so large that MAS is not appropriate.2M This is because the maximum achievable spinning rate is less than the linewidth due to the second-order quadrupole interaction. The quadrupole coupling constants for the 71Ga sites were estimated to be of the order of 12.8MHz from the static spectra.206 Changes in the 31P, I9F (fluorine is present from the synthesis conditions) and 'H NMR spectra with temperature have also been reported.204 An example of the use of NMR was recently provided by the structural determination of the titanosilicate ETS-10 by Anderson et a1.*08 This material was discovered in 1989 and has only been synthesized with a very small particle size (so single crystal techniques are inappropriate) and exhibits significant long-range disorder. The structure consists of cornersharing S i 04 tetrahedra and T i 0 6 octahedra linked through bridging oxygen atoms. A crucial aspect in the structure determination was establishing the local silicon environment by 29Si NMR, which could then be combined with electron microscopy and powder diffraction results to propose a structure. Figure 11 shows the 29Si MAS NMR spectrum of ETS-10 which shows that there are four Si(3Si,lTi) environments (two of which virtually overlap) and one Si(4Si) environment, and these can be related to the silicon locations in 12-membered rings or seven-membered rings of ETS-10. The pore system consists of 12-membered rings (size 7.6 X 4.9 A), and can be subject to considerable disorder. It is interesting to note that some of the ordered variants of ETS-10 are chira1.208

2.3.2. Layered microporous materials There are also a wide variety of microporous materials based on layered materials. Pillared and intercalated clays have been known for many years, and many have potential as catalysts.2w It is also possible to create microporous layered materials from phosphates, titanates and metal oxides using a variety of pillar^.^^^^^^ Early common pillaring agents were large organic cations, though these tend to have low thermal stability, and alumina and silica species. The use of more complex pillaring agents including organosilicon compounds such as NH2(CH&Si( OCzH5)3, the

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Si(3Si. ITi) 2

Fig. 11. 29Si MAS NMR spectrum of titanosilicate ETS-10. LReproduced with permission from M. W. Anderson et al., Nature, 1994, 367, 347.)’ *

aluminium Keggin ion [A11304(OH)24(H20)12]7+ and the zirconium tetramer [Zr(OH)2(H20)4],8f have led to microporous materials with relatively high thermal stabilities. Particular areas of importance are the composition of the intercalate before and after calcination, and the nature of the binding of the pillar to the layers. In the case of clays intercalated with the AlI3 complex it has been shown that the location of the charge in the clay has a critical effect in determining whether or not cross-linking occurs at high Recently an NMR study of pillared saponite has suggested that alumina-like pillars cross-link to silicon sites, while silica-like pillars in contrast may cross-link to aluminium sites in the layers.217 One of the most studied layer phosphate materials is ( Y - Z ~ ( H P O ~ ) ~ . H , O , which may readily be intercalated with a variety of compounds to give very high surface area materials. This is a particularly interesting system as there has been a report of a chiral intercalate binding with a significant enantiomeric excess, thus giving the possibility of chiral recognition.218 31P MAS NMR spectra show a number of different phosphorus environments within the layers upon intercalation of a-Zr(HPO,J2.H2O with various amines, and the chemical shifts reflect the extent of hydrogen bonding between the layers and the The changes with temperature that occur when a-Zr(HP04)2.H20is refluxed with NH2(CH2)3Si(OC2H5)3have been examined in detail by 29Si NMR (Fig. 12).220 The 29Si MAS NMR


65

-67.5

0

-80

-160

6 /PPm Fig. 12. 29Si MAS NMR spectra of ( Y - Z ~ ( H P O ~ ) ~ after . H ~ Ointercalation with NH2(CH2),Si(OC2H5),: (a) uncalcined sample, (b) after calcination at 450°C for 2 h and (c) after calcination at 600°C for 2 h. (Reproduced with permission from L. Li et al., J . Phys. Chem., 1991, 95, 5910. 01991, American Chemical Society.)’”

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spectrum of the uncalcined sample shows major peaks at -59.2 and -67.5ppm due to Q2and Q3 organosilicate units respectively, indicating that the organosilicon compound has undergone polymerization and partial dehydroxylation during intercalation into the interlayer space. Upon calcination at 450°C or 6OO0C, the organic moiety of the intercalate decomposes and Q 2 , Q3 and Q4 species of a silica-like species are produced. The pillared material is stable to over 700°C and can sorb species such as n-hexane.220 The 31P and 13C chemical shift tensors in closely related zirconium phosphonates have recently been determined,221and used to determine the geometry of the organic group in uniaxially oriented films of Zr(03PCH2COOH)2.222 27Al MAS NMR is a useful probe of the state of aluminium polyoxycations in the interlayer space. Thus the AlI3 Keggin ion intercalated in a layered titanate, for example, gives tetrahedral and octahedral 27Al signals in the approximate ratio 1:12 as expected.223 Upon calcination to 500°C there is dehydration and significant dehydroxylation to give a species giving a 27Al spectrum similar to that of y - a l ~ m i n a The . ~ ~ 27Al ~ spectrum from aluminium polyoxycations may, however, be extremely complicated as observed, for instance, in the case of the AIl3 Keggin ion intercalated into the layered lattice of Moo3. Here a range of different aluminium environments, with different chemical shifts and quadrupole coupling constants, are observed, indicating a change in the complex structure on intercalation with possible binding to the molybdenum layers even before calcination.224

3. NMR STUDIES OF MESOPOROUS MATERIALS

3.1. Silicas, aluminas, etc,

Mesoporous materials are of considerable importance as sorbents and catalyst supports. Unlike most of the microporous materials discussed above, these materials are not crystalline and thus have no well-defined pore structure. As a consequence, they give rise to fairly broad NMR peaks, reflecting the range of local environments present. 3.1.1. N M R of silicas

Silicas may be subdivided into solution-based silica gels and into pyrogenic silicas. The first category are porous materials, and the pore dimensions depend on the exact method of preparation. They can contain both micropores as well as mesopores and can have very high surface areas. Pyrogenic silicas on the other hand, typified by Aerosil, are non-porous.


67

Q4

-00

- 100

- 120

6 /PPm Fig. 13. 2ySi NMR spectra of a silica gel: (a) MAS (3min recycle delay) and (b) CPiMAS (10 ms contact time, 2 s recycle delay).

Silicas have been widely studied by NMR since the pioneering work of Maciel and S i n d ~ r f . ~ ~ ~ Figure 13 shows typical 29Si MAS and CP/MAS NMR spectra of a silica gel. Signals are observed from Q2, Q3 and Q4 species at about -90, -100 and -110ppm respectively (where Q" denotes a silicon bonded to four oxygens, n of which are bridging). These correspond to geminal hydroxyl &(OH), single hydroxyl silanol groups, (=SiO)3 silanol groups %(OH), and bulk (=SiO)4&i species respectively. Provided that sufficient time is left between scans for relaxation (this normally needs to be at least

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2 min) the spectrum in quantitative, and spectral deconvolution allows the relative amounts of these species to be obtained. The small amount of geminal groups means that measurement of their proportion by this method is particularly suspect to error. The single pulse experiment does, however, give a reliable measurement of the fraction of Q2+ Q3 environments present. The 2’Si CP/MAS NMR spectrum, on the other hand, discriminates against Q4 sites as these have no nearby hydrogens from which to cross-polarize, while the Q2 and Q3 sites readily give a strong signal. The rates of cross-polarization of the Q2 and Q3 sites are normally fairly similar, and thus CP spectra at contact times greater than about 5 m s provide a reliable measurement of the relative amounts of Q2 and Q3 sites present. For more accurate measurements, a variable contact time study may be performed in order to correct intensities for CP dynamics e f f e ~ t s . ~Thu ~ ~S, ~ ~ ’ the combination of single pulse MAS and CP/MAS experiments on silica gels readily gives a quantitative analysis of the Q2, Q3 and Q4 sites present. Chemical changes at the silica surface during dehydration and rehydration are particularly important areas of study. Spectra of dehydrated samples give significantly larger linewidths due to the dispersion of chemical shifts in the absence of mobile molecular ~ a t e r . Reversible ~ ~ ~ , condensation ~ ~ ~ of hydroxyl groups is believed to occur at temperatures up to about 500”C, after which rehydration becomes increasingly difficult. Some studies have observed an initial decrease in the relative proportion of geminal hydroxyl sites during dehydration, followed by an increase in their relative proportion at higher temperatures indicating that complex chemical changes must be occurring at the surface.228On the other hand, another study found a similar proportion of geminal hydroxyl groups for a range of samples at various stages of dehydration and rehydration .227 Another question of interest is the nature and the number of interior hydroxyl groups which are not readily accessible. Hydroxyl groups at the surface may be fully deuterated after several cycles of exchange with D 2 0 (unless previously subjected to high temperatures). 2ySi CP/MAS NMR spectra on deuterated samples have recently shown that there are negligible amounts of internal geminal hydroxyl groups, and have measured the number of internal single hydroxyl groups.22y On the basis of the rates of cross-polarization for the internal single hydroxyl groups, and estimates of the dipolar coupling to hydrogen, it appears that the internal single silanols undergo rapid rotation about the Si-OH bond axis.22y The surface of silica gel may also be studied by ‘H NMR. In order to achieve good resolution it is necessary to remove ‘H-lH dipolar interactions either by extremely fast MAS, or by combining MAS with multiple-pulse line narrowing methods (CRAMPS). Some ‘H CRAMPS spectra are shown in Fig. 14.230The untreated sample shows two narrow signals and one broad one. The narrow peak at 3.5ppm is removed upon evacuation at room temperature and hence can be assigned to physisorbed water. The broad

(a2)


69

25°C Evac.

5 0 0 ° C Evac.

Fig. 14. 'H CRAMPS spectra of a silica gel: (a) untreated sample, (b), (c) and (d) after evacuation at the temperature indicated. (Reproduced with permission from C . E. Bronnimann et al., J . Am. Chem. Soc., 1988, 110, 2023. @ 1988, American Chemical Society.)230

peak centred at about 3.0 ppm is removed upon evacuation of the sample at 5OO0C, leaving only the narrow peak at 1.7ppm which may be assigned to isolated Si-OH silanol species. The broad peak is probably due to hydrogen-bonded silanol species, and the linewidth of this peak reflects a range of different chemical environments and hydrogen bond strengths. There are several methods of modifying the surface of silica. The hydroxyl

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groups may be replaced by methyl groups by treatment with methylchlorosilanes; this treatment completely changes the properties of the silica by making the surface hydrophobic. 29SiNMR peaks for the methylsilyl groups at the surface may readily be detected using CP/MAS, and a range of different environments may be seen depending on the extent of any hydrolysis or additional condensation reactions at the surface.231 Other silylating reactions have also been explored in detai1.232-234Other treatments include modification of surface acidity by introducing phosphorus, and of course metal and metal ions may be deposited on the surface, as is common for heterogeneous catalysts. 3.1.2. N M R of aluminas

Alumina is the other common catalyst support, and different forms of alumina can have widely different surface areas, and different strength Bronsted and Lewis acid sites. 27Al NMR is handicapped by the broad range of environments present coupled with the complications arising from the second-order quadrupole interaction. However, it is still straightforward to identify octahedral and tetrahedral aluminium species. Thus NMR can readily distinguish between a-alumina, which contains only octahedral aluminium, and y-alumina, which has a quarter of aluminium in tetrahedral sites. It is interesting to note that (static) 27Al NMR spectra of alumina have been observed recently using a field-swept NMR spectrometer at 15 T magnetic field strength and 4.2 K.235 One anomaly that has been observed in the 27Al MAS NMR spectra of aluminas is an apparent decrease in signal intensity with increasing surface area of sample. It has been suggested that this is a result of the surface layers experiencing very large electric field gradients and so being broadened beyond detection.236 Recently, however, results have indicated that while the loss in 27Al signal with increasing surface area is a real phenomenon, it is due to dynamic effects at the surface involving proton migration, and that surface aluminium can indeed be observed under appropriate conditions.237 One possible method of gaining surface selectivity in an NMR experiment is using cross-polarization to transfer magnetization from surface hydroxyl protons to give the 27Al spectrum. This successfully detects octahedral surface aluminium species and the CP intensity is diminished when the sample is subjected to any dehydroxylation t r e a t r n e n t ~ . ’ ~ However, ~ no tetrahedral Bronsted acid sites were detected, either because of long AI-H distances or complications arising from cross-polarizing quadrupolar nuclei under MAS condition^.^^ 27Al CP/MAS also has the potential to investigate Lewis acid sites at the surface as well as B r ~ n s t e dsites. The surface hydroxyls may be fully deuterated, and then a base such as pyridine adsorbed. In this case all the 27Al detected under CP conditions will come from those aluminium sites very close to bonded pyridine molecules.238


71

Probe molecules adsorbed on alumina may also be studied in the same way as probe molecules on zeolites. For instance, I5N MAS NMR may be profitably used to study ”N-enriched pyridine bonded to acid sites on the surface of y-alumina. Two resonances are observed, and dipolar dephasing experiments show that neither comes from a hydrogen-bonded or protonated pyridine molecule, indicating that there are two distinct Lewis acid It has been suggested that these arise from occupation of tetrahedral and octahedral anion vacancies on the surface.”’ The I5N shielding anisotropy of both sites could be obtained by simulating the MAS spinning sideband intensity pattern, and this gives a lower anisotropy than for solid pyridine, indicating that there is a degree of molecular motion. Static 2H experiments on deuterated pyridine indicate that there is a heterogeneous distribution of pyridine motions, and that major contributions arise from continuous diffusion or C2 flips about the twofold symmetry axis.239 Multinuclear NMR experiments have also been performed on amorphous silica-alumina catalyst supports, with results that are broadly similar to those expected from a combination of silica and alumina.24c243 The 29Si NMR spectra tend to be very broad and reflect a range of environments: the S i 0 4 tetrahedra can be essentially randomly distributed in samples prepared by coprecipitation, and there is also evidence for single silanol and geminal hydroxyl silanol 3.1.3. N M R us u probe of pore size

A recent elegant use of NMR on porous solids is to gain information on pore size. It is well known that the freezing temperature of a liquid within a pore is reduced by an amount which depends on the pore diameter. Static ‘H NMR spectroscopy can quantitatively measure the relative amounts of solid and liquid present. Thus cyclohexane can be adsorbed by silica and spectra obtained as a function of temperature using a spin-echo pulse sequence that has the effect of suppressing the signal from solid cyclohexane. Cyclohexane is a particularly favourable liquid to use as it shows a large melting point variation and forms a soft plastic crystalline phase on freezing that appears not to damage pore structure. The amount of liquid cyclohexane present as a function of temperature can then be used to calculate a pore size distribution plot.244 Figure 15 shows the results of this NMR analysis using cyclohexane on a porous silica with a nominal pore diameter of 200 A. Good agreement is observed with the results of gas adsorption and desorption measurements. The freezing of water adsorbed on porous solids including silica and activated charcoal has also been explored. Here there may be two distinct freezing processes: strong bonding to the surface, and normal freezing which occurs at a temperature predicted by the Kelvin equation and so depends on pore size. It has been suggested that

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Pore diameter / A Fig. 15. Pore size distribution of a silica with nominal pore diameter of 200 A. The solid line gives that determined from 'H NMR spectra of adsorbed cyclohexane, while the points give that determined by conventional gas adsorptioddesorption measurements. (Reproduced with permission from J. H. Strange et al., Phys. Rev. Lett., 1993, 71, 3589. @ 1993, American Physical Society.)244

measurements of water freezing with temperature can provide a estimate for the true pore volume than other techniques such mercury-intrusion method.245 Pore geometries may also be studied by relaxation time Analysis of relaxation curves can give pore size distributions, knowledge of surface relaxation interactions is needed to gain information, and the method is not always reliable.2473248

better as the

but a useful

3.2. Novel mesoporous materials

3.2.1. MCM-41 There has recently been considerable interest in a new family of mesoporous materials, designated MCM-41, which possess regular hexagonal arrays of uniform channels between 16 and 100 in diameter. Cubic phase mesopor-


73

ous analogues have also recently been prepared. MCM-41 is synthesized from aluminosilicate gels in the presence of quaternary ammonium surfactants, and appears to form by a liquid-crystal templating mechanism in which the silicate material forms inorganic walls around ordered surfactant r n i c e l l e ~ The . ~ ~pore ~ ~ ~diameter ~~ depends principally on the alkyl chain length of the quaternary ammonium surfactant used in the synthesis. The pore structure remains intact even at temperatures as high as 8OO"C, and MCM-41 can exhibit pore condensation without h y ~ t e r e s i s . ~As ' ~ ,well ~ ~ ~as its distinctive adsorption properties, MCM-41 materials have great possibilities as high surface area supports and potential applications in the preparation of nanosized particles. 13C MAS NMR studies of the asprepared material show narrow peaks, and only a very weak CP signal of the hydrocarbon end of the surfactant can be observed. This indicates that there is significant molecular motion, particularly at the hydrocarbon end, as anticipated for a micellar array with the quaternary ammonium end linked to the silicate/aluminosilicate wall and the hydrocarbon end free in the middle of the channels.25o29Si MAS NMR spectra of siliceous MCM-41 give a broad spectrum similar to amorphous and 29Si NMR has already shown that the hydroxyl surface groups may be functionalized by treatment with trimethylchloro~ilane.~~~~~'~ Recently, 27A1 NMR has confirmed that it is possible to synthesize MCM-41 with fairly high levels of aluminium, all of which are tetrahedrally coordinated in the framework.25s

3.2.2. Organosilicates There have recently been preparations of porous organosilicate materials consisting of hybrids between organic and inorganic networks. Thus hydrolysis and condensation of monomers such as (Et0)3Si-Ar-Si(OEt)3 by sol-gel processing forms polysilsesquioxane ~ e r o g e l s These . ~ ~ ~ novel materials exhibit both microporosity and mesoporosity and can have surface areas as high as 1000m2g-'. 29Si CP/MAS NMR spectroscopy of these organosilicates shows peaks due to Q', Q2 and Q3 organosilicon units. Quantification of these peaks allows the determination of the degree of condensation that has occurred, which depends on the exact synthesis Chlorosiconditions used and the size of the organic building lane precursors have also successfully been used and the products studied by "SSi NMR.257

3.3. Catalysts

There have been a large number of NMR investigations into heterogeneous catalyst systems supported on silica and/or alumina. It is not possible to discuss the wealth of useful NMR data recorded on these systems, but NMR

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has been used to study the surface of the catalyst with and without adsorbates.25a260 A recent advance in using NMR to study catalytic systems is the use of dipolar coupling between labelled I3C nuclei to give useful information including measurements of accurate bond lengths.2613262 Thus in the study of benzene on a Pt/q-A1203catalyst, a I3C dipolar study of doubly 13C-labelled benzene shows a single C-C bond length, implying the absence of a significant distortion of benzene which had been previously The dynamics of the benzene molecule were also explored and ring reorientation was found to occur even at temperatures as low as 6 K in contrast to bulk b e n ~ e n e . *In ~~ the ? ~same ~ ~ system, static *H spectra have been used to explore dipolar coupling to 19’Pt, and the structure in the spectrum indicates that the benzene adsorbs directly over a platinum atom. The coupling magnitude suggests that the Pt-benzene distance is 1.56 A; however, there will also be a contribution from pseudodipolar coupling which would increase this Another method of gaining structural information is to measure I3C--lH dipolar coupling using a SEDOR pulse sequence as used, for example, in studies of acetylene on platinum and other metal Such studies of adsorbed species on catalysts by NMR are highly dependent on the detailed nature of the catalyst (metal particle size and distribution, nature of the support, how ‘‘clean’’ the surface is before adsorption) and it is not uncommon for different research groups to obtain slightly different results. Also, it is not always possible to record good quality I3C CP/MAS spectra of chemisorbed species on metals due to metal susceptibility problems. For instance, ethylene on Ptly-A1203 gives only broad 13C CP/MAS peaks. However, the deuterated form could still be studied by static 2H NMR revealing that the dominant chemisorbed species was ethylidyne ( E C - C H ~ ) On . ~ ~the ~ other hand, I3C CP/MAS spectra of ethylene adsorbed on Ag/y-A1203 show that the major species is .rr-bonded ethylene with the C=C axis parallel to the silver surface. In this case the I3C shielding anisotropy and C-C distance have been measured by recording spectra at low temperature with the use of isotopic labelling.269At high temperature the dynamics of the weakly-bonded ethylene on the silver surface have also been e ~ p l o r e d . ~ In ” a I3C CP/MAS NMR study of ethylene on oxygen-covered Ag/77-A1203 a range of other peaks may be observed corresponding to carboxylate species, and these may be inferred to be intermediates in the complete combustion of ethylene.271 As well as studies on the geometry and dynamics of adsorbates, NMR has been used to follow reactions in situ in a similar fashion to that described for zeolites. Reactions studied include commercially important ones such as methanol synthesis over Cu/ZnO/AI2O3 catalysts,272and acetylene cyclotrimerization over Pt/A1203systems.273Probe molecules may also be useful, such as using 31P NMR of adsorbed trimethylphosphine to determine acid site concentrations on a commercial silica-alumina cracking catalyst.274


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4. lz9Xe NMR STUDIES OF POROUS MATERIALS

4.1. ‘29Xe NMR of zeolites Pioneering 12’Xe NMR work by Ripmeester and co-workers on xenon trapped in ~ l a t h r a t e s and ~ ~ ~by. ~Fraissard ~~ and co-workers on the behaviour of ‘”Xe chemical shifts inside zeolites277has led to a considerable interest in using 12’Xe NMR spectroscopy as a sensitive inert probe of pore structure and a number of reviews have The major feature of the technique is that the 12’Xe NMR chemical shift of xenon inside microporous solids is intrinsically affected by pore size as well as xenon gas pressure, the presence of cations and possible strong adsorption sites. It was demonstrated early on that the I2’Xe chemical shift in zeolites Na-Y and H-Y varies approximately linearly with xenon concentration,277 and only has a small dependence on framework composition (%/A1 Other zeolite structures show similar behaviour but with a different value for the chemical shift extrapolated back to zero xenon concentration, here termed as (at which point there will be no effects from xenon-xenon collisions).283For instance isostructural A1P04-5, SAPO-5 and the purely siliceous analogue, SSZ-24, have identical Ss values, but different from that of zeolite Na-Y.284 It is generally found that the larger the dimensions of the cavity containing xenon, the smaller the value of 6s observed. Several empirical correlations of as against pore size were suggested but none of these is entirely sati~factory,~” and simple model calculations have indicated that no simple correlation is likely and that the temperature dependence must also be considered.285 The behaviour of 12’Xe chemical shifts for xenon inside zeolite Y with other cations is in fact rather more complicated than described in the previous paragraph. First of all, if large univalent cations are present, as in the case of zeolites K-Y and Rb-Y, then higher values of Ss than expected are observed, possibly because the cation reduces some of the effective pore volume.286 In the presence of divalent cations, such as Ca2+ and Mg2+, in zeolite Y there is further anomalous behaviour, as it is found that the 12’Xe chemical shift increases as the xenon concentration is reduced.286 This has been rationalized in different ways as being due to a distortion of the xenon electron cloud by strong electric fields278,286or to the presence of strong adsorption sites on the cations.287 More recently it has been found that in zeolites X and Y containing Ag+ or Cu+ cations a significant reduction in chemical shift occurs as the xenon concentration is reduced, in contrast to the behaviour with other univalent cations, or multivalent i ~ n s . ~ ~This ~ ’ is ’ illustrated in Fig. 16 which shows the ‘*’Xe chemical shift variation with xenon concentration for Cd2+-, Zn2+-, Na+-, Cu+- and Ag+-exchanged zeolite X. With the exception of Na+, all these cations have dl’ configurations. It has been argued that the reduction in chemical shift at lower

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0

5E+20

IE+21

N (Xe atoms /g 1 Fig. 16. Plot of '"Xe

chemical shift against xenon concentration for zeolite X containing different cations. (Reproduced with ermission from A. Gedeon and J. Fraissard, Chem. Phys. Lett., 1994, 219, 440.)2 9 9

pressures for the Cu+ and Ag' cases is due to a specific d'"-d" .rr-back donation from the metal to the xenon atom.288-290However, this interaction does not occur, or is masked by other effects, for the divalent d" cations. Thus it seems that the NMR behaviour of 129Xein the proximity to different cations is not yet fully understood. There will also be additional effects on the '29Xe chemical shift if the cation is paramagnetic. Despite this, there have been a number of studies using lZ9XeNMR to probe cation locations and metal clusters in zeolites with a certain degree of success.291-304 12'Xe NMR may also be useful in identifying more than one adsorption zone inside a zeolite as in the case of mordenite, offretite and zeolite rho,


77

provided that xenon exchange between the zones is not fast on the chemical shift t i m e s ~ a l e . ~ ~ ~ - ~ ~ ' One particular application to which '29Xe NMR is well suited is probing the homogeneity of the sample. For instance, upon adsorption of water into dehydrated zeolite Na-Y containing xenon, more than one 129XeNMR peak is observed as initially the water adsorbs strongly into exposed crystallites, leaving others bare. A single narrow '29Xe NMR peak can be observed after shaking or heating the sample to produce a homogeneous distribution of water.308 Indeed the change in lz9Xe NMR spectrum with time may be used to measure the rate of water d i f f u s i ~ n Similar . ~ ~ heterogeneous distributions of organic adsorbates in zeolites have also been o b ~ e r v e d . ~ ~ ~ , ~ ~ ~ One crucial aspect in the interpretation of 129Xe NMR spectra is the volume over which the xenon atoms undergo fast exchange on the NMR chemical shift timescale. This may be a single cavity (as discussed for zeolite Na-A below), many cavities or even intercrystalline. For instance, two distinct species are observed at room temperature for xenon adsorbed in a mixture of zeolite Na-Y and Ca-Y, but only a single average peak is observed upon thorough mixing of the different zeolite forms due to fast intercrystalline diffusion unless the temperature is lowered.312 Other examples of fast interparticle exchange in '29Xe NMR spectra have also been o b ~ e r v e d . " ~Because of this, the packing density of a powder sample can also have a significant effect on '29Xe NMR spectra.314 Thus 129Xe NMR may give both microscopic and macroscopic information depending on the system under investigation making useful spectral interpretation particularly difficult. In some cases it is helpful to cool the sample in order to slow down the diffusional processes occurring when microscopic information is desired, though this might of course also affect the chemical shift by altering the rate of exchange between xenon adsorbed at the surface and in the free volume.315 In the case of xenon adsorbed in zeolite Na-A, the window between a-cages is about 4.2 A in Na-A, which is slightly smaller than the diameter of xenon atoms (approximately 4.4 A). Xenon can, however, be forced into the cages of zeolite Na-A at elevated pressures and temperatures. Differing xenon concentrations can be obtained depending on the pressure applied. In this case diffusion between cages is slow on the chemical shift timescale and so this is a good model system for attempting to understand '29Xe chemical shift behaviour in more detail. Distinct peaks are observed corresponding to differing numbers of xenon atoms, n , present in each cage (Fig. 17).316318 Hence the distribution of xenon atoms in the a-cages may be measured. Interestingly the equilibrium distribution (which is reached after many days, though this may be shortened by heating) is found to deviate from random statistical models, indicating that there are attractive xenon-xenon interactions which favour clustering at low to medium loadings, together with higher energies associated with overcrowding of cages at higher loadings

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2

I

300

1

I

200

100 6 /PPm

I

0

Fig. 17. Xenon in zeolite Na-A occluded at 523 K at different equilibrium pressures: (a) 8 atm, (b) 40 atm, (c) 150 atm and (d) 210 atm. The numbers above the peaks indicate how many xenon atoms are in the a-cage giving rise to the peak. (Reproduced with permission from B. F. Chmelka et a l . , Phys. Rev. Lett., 1991, 66,

580. 01991, American Physical Society.)317

(particularly cages containing seven or eight atoms) .318 The overcrowding of the cages is also indicated by the observed chemical shifts: the difference in chemical shift between adjacent peaks increases slightly from 17.5 pprn (between n = 1 to n = 2) to 25.1ppm (between n = 5 to n = 6) at which point there is a large increase of 45.1 ppm between the n = 6 and n = 7 chemical shifts, and a further 43.7 ppm increase to the n = 8 resonance. The temperature dependences of the chemical shifts have also been explored.318 Interestingly '29Xe peaks at the same positions may be observed even after the adsorption of a small amount of water, together with an additional peak


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at 185 ppm, indicating once again that adsorption is not necessarily homogeneous.319 In an attempt to understand the chemical shift behaviour, ab initio calculations on adsorbed 39Ar atoms have been performed, and the correspondence with the observed lz9Xe NMR data suggest that the theoretical aspects are gradually becoming better u n d e r ~ t o o d . ~While ~ " the rate of exchange between xenon atoms in neighbouring cages is slow on the chemical shift timescale, it still may be measured by NMR using twodimensional exchange spectroscopy. Cross-peaks are obtained as a function of mixing time, and these give a direct measure of the mass transport of xenon between the cages.321 It is found that the relative xenon adsorption separation energies are relatively constant for occupancies up to four xenon atoms in a cage, after which the separation energies decrease due to repulsive interactions as the cages become more crowded.321 It is interesting to note that in zeolite Ca-A the Ca2+ ions are not located near the windows between cages, and xenon can easily diffuse into neighbouring cages. Hence only a single 129Xepeak is observed in the NMR spectrum. Observations at high pressures have shown that the chemical shift does not vary linearly with xenon concentration, but depends on the detailed nature of the pore structure.322

4.2. '29Xe NMR of other porous solids There is a significant difference in the 12'Xe NMR chemical shift behaviour for mesoporous materials compared to microporous solids. It has been found that at room temperature the lZ9Xe chemical shift is approximately independent of xenon pressure for a range of silica gels.323 This has also been observed for xenon adsorbed in the pore structure created by compressing non-porous silica spheres.324 The observed chemical shift is comparable to that in zeolites, and reflects fast exchange between surface adsorbed and free xenon. An empirical correlation between the chemical shift and the average pore diameter has been suggested.323 At low temperatures the lZ9Xe NMR chemical shift does show a dependence on xenon c o n c e n t r a t i ~ nThis . ~ ~ dependence ~ is not, however, straightforward as there will be preferential adsorption of xenon into any micropores present; this will manifest itself as an increase in '29Xe chemical shift as the xenon concentration is reduced.325 One particular feature of lZ9Xe measurements on small amorphous particles is that the particle size may have a significant effect. Figure 18 shows lZ9XeNMR spectra obtained on a silica gel after grinding in a mortar. A broad peak is observed, unless all small particles are removed, in which case narrow peaks corresponding to adsorbed and free xenon gas are observed.323 This is probably due to the small particles having a range of free volumes and interparticle exchange effects becoming more important.

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65

(a)

t

200

I

*

1

100 0 WPPm

1

1

-100

Fig. 18. 129XeNMR spectra of adsorbed xenon in a silica gel: (a) after grinding the silica, (b) after removing all small particles leaving those with sizes in the range 150-250~m. (Reproduced with permission from V. V. Terskikh et al., J. Chem. SOC., Faraday Trans., 1993, 89, 4239.)323

Complicated broadening effects due to small particles have also been observed in other systems,314and it is clear that this factor has to be borne in mind when interpreting spectra. In principle, 129XeNMR spectra of porous layered materials should be affected by similar considerations to those described for zeolites,326 but another macroscopic factor must also be considered. Different preferred orientations of a powdered sample can give different 129XeNMR chemical shifts as is illustrated for a montmorillonite clay in Fig. 19, where vastly different spectra are obtained depending on the alignment of the clay A number of other porous systems have been studied by the lZ9XeNMR technique. It has been applied to study porous carbons, either as an attempt to gain information on pore dimension^,^^' or to observe the effect of acidic functional groups on the surface.328It has been used to probe the amount of hydrogen adsorption on platinum particles supported on alumina or other supports which is of importance in catalysis.3293330 It has also been used to confirm microporosity in certain heteropolyoxometallates.331One last application of 129XeNMR is two-dimensional exchange spectroscopy, which may be used to probe the rate of exchange between adsorbed xenon and free xenon gas, giving information on adsorptioddesorption rates.332


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aligned platelets

Fig. 19. lzYXeNMR spectra of xenon adsorbed in tetramethylammonium-exchanged montmorillonite: (a) powder spectrum, (b) and (c) spectra for platelets aligned parallel and perpendicular to the magnetic field. (Reproduced with permission from J. A. Ripmeester and C. I . Ratcliffe, Anal. Chim. Acta, 1993, 283, 1103.)314

It is thus clear from this discussion that there are a considerable number of factors, both microscopic and macroscopic, that can influence '29Xe NMR spectra of xenon adsorbed in porous solids and it is dangerous to draw conclusions from spectra on unknown systems without additional information. The theoretical factors behind '29Xe NMR chemical shifts are, however, gradually becoming better understood, and already '29Xe NMR spectroscopy is a useful probe of the homogeneity of porous systems.

5. NMR STUDIES OF MOLECULAR TRANSPORT IN POROUS SOLIDS

Various methods have been mentioned earlier of characterizing pore geometry by the influence on chemical shifts and relaxation times. In this section some experiments to measure the diffusion of adsorbed species within porous solids are discussed; these will give additional information on

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pore geometries, and on the important process of mass transport within porous solids.

5.1. Pulsed field gradient measurements

Diffusion measurements and the study of mass transport are particularly important in the understanding of catalysis. The possibility of interpreting and correlating molecular mass-transfer phenomena in solids depends critically on a knowledge of the surface structure and its regularity. In this respect, diffusion in zeolites and related materials are ideal systems to study as zeolites have well-defined pore structures. There have been a large number of studies on hydrocarbon diffusion in zeolites, particularly by Karger and co-workers, and these have recently been reviewed.3333334 Diffusion measurements on adsorbed species can be made by a variety of NMR methods normally based upon spin-echo pulse sequences.33s 'H or *H spin-lattice relaxation time measurements may also give diffusion coefficients in favourable circumstances. lo' The first NMR self-diffusion measurements of adsorbate-adsorbent systems were carried out in 1967 using the constant field spin-echo method, and it was found that methane adsorbed on silica gel had a similar diffusion rate to that of the free In contrast, results on systems where the pore diameters only slightly exceed the size of the adsorbate molecules show that self-diffusion coefficients are commonly lower by more than two orders of magnitude than in neat The current NMR method of choice to measure diffusion coefficients is a pulsed field gradient (PFG) spin-echo t e ~ h n i q ~ e , ~ ~ ~ , though alternative techniques are still being devised.339 In PFG NMR, magnetization is first dephased and then subsequently rephased under the influence of a spatially dependent magnetic field gradient. Observing the NMR signal intensity as a function of the applied field gradient pulses provides direct information about the diffusion of individual molecules. A major advantage of the NMR method of measuring diffusion rates is that it is a measurement performed under equilibrium conditions and so there is no interference from intrinsic mass transfer effects. A feature of the PFG technique is that it measures diffusion over distances of the order of a few micrometres. In the case of diffusion in zeolites, this is normally less than the size of the crystallites and we are thus observing only intracrystalline self-diffusion, without any complications from boundary or interparticle effects.340Cases of restricted self-diffusion, where molecular propagation is comparable with crystal size, and long-range self-diffusion, in which molecular propagation occurs over many crystallites during the measurement, may also be identified unambiguously by PFG NMR.340 Many PFG NMR experiments have measured diffusion coefficients which are up to five orders of magnitude higher than values obtained by traditional


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uptake methods. Such discrepancies are predominantly due to surface effects and interparticle transport processes affecting the values obtained by the sorption technique. PFG measurements may easily be checked for reproducibility as they are obtained on an equilibrium system, while sorption methods require repetition of the complete desorption/adsorption cycle to check the reproducibility of the measurement, and many of the discrepancies between PFG NMR and uptake measurements have now been removed by careful remeasurement of the sorption data. It should also be observed that the self-diffusion measurements obtained by the two techniques are not necessarily equivalent, as one represents an equilibrium measurement (on a system in which the structure of the solid may have been slightly altered by the presence of the adsorbate), while the other is a non-equilibrium measurement (reflecting adsorption into regions of the solid containing no adsorbate). It should also be borne in mind that PFG NMR measurements are normally performed at low temperatures and comparatively high loadings, and molecule-molecule collisions may have an Other methods for measuring intracrystalline self-diffusion include molecular dynamics calculations341 and quasi-elastic neutron ~ c a t t e r i n g , ~and ' ~ these give results very similar to those of PFG NMR. Both macroscopic and microscopic diffusion rates can be obtained by an NMR tracer exchange method in which the rate of exchange between protoncontaining molecules in the gas phase and deuterated adsorbed molecules is followed by 'H NMR, and then the sample studied by PFG NMR after attaining equilibrium. 333 One particularly interesting application of PFG NMR is that it can measure diffusion anisotropies for non-aligned crystallites by analysis of the shape of NMR signal attenuation as a function of the gradient amplitude. For instance, it has been used to estimate that for methane in zeolite ZSM-5, the diffusivities in the different channels present differ from each This measurement of diffusion anisotropy other by a factor of about is in good agreement with results obtained on oriented ZSM-5 crystallites. PFG NMR experiments at high temperature can allow diffusivities to be obtained that would otherwise be too slow to measure accurately by the technique, and it is found that the diffusivities of n-alkanes in ZSM-5 decrease monotonically with increasing chain length.344 Such studies have shown that the Si/Al ratio of ZSM-5 has no effect on the diffusion of n - a l k a n e ~ . ~ ' ~On . ~ 'the ~ other hand, the rates of water and benzene diffusion are affected, presumably because of interactions of the adsorbate with the charge-balancing ~ a t i o n s . ~While ~ ~ , ~'H ' ~nuclei have been most frequently studied by PFG NMR due to sensitivity reasons, other nuclei may also be measured. Thus I3C PFG NMR has recently been used to study CO and C 0 2 diffusion in zeolites.348 Another advantage of the PFG NMR technique is that it is possible to measure the diffusivities of different species in a multicomponent system if

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they have different chemical shifts. This has been demonstrated by the simultaneous measurement of the diffusivities of ethane and ethylene adsorbed in zeolite Na-X.349 It was found that ethylene has a smaller diffusivity by a factor of three to four, reflecting the interactions between unsaturated hydrocarbons and sodium cations, but it should be noted that this difference is significantly smaller than that observed if the components are present on their own. This implies that the mutual interaction between the two adsorbates is significant and shows that diffusivities in multicomponent systems, such as occur in catalytic conversions, are not necessarily the same as those measured for single-component systems. Another multicomponent system that has been studied is cyclopropane and propene in zeolite X.350 Here the catalytic conversion of cyclopropane to propene has been studied in sifu by PFG NMR, and it was found that the diffusivities were sufficiently large for this system that any effects of intracrystalline diffusion in a flow reactor could be n e g l e ~ t e d . ~ ” Diffusion in amorphous solids, such as silica gel, is more complicated than in zeolites due to variations in pore size, but it can still be measured by PFG NMR. In experiments on n-alkanes on silica gel, two distinct diffusion processes could be observed during the observation time, and these correspond to surface self-diffusion (associated with microporous regions) and long-range self-diffusion (associated with the mesoporous regions between particles) .351 Models describing the influence of pore morphology on PFG NMR studies of fluid-saturated porous solids including restricted diffusion measurements are gradually being d e ~ e l o p e d . ~ ~ * - ~ ’ ~ 5.2. Applications of NMR imaging

In many cases one is interested in diffusion over longer time periods than accessible by PFG NMR studies. These may now be measured by following the concentration of molecules with distance in real time using NMR imaging. Thus the transport of water from saturated to unsaturated parts of porous systems may be followed q ~ a n t i t a t i v e l y . ~It’ ~is~also ~ ~ possible to follow the diffusion of paramagnetic ions such as Ni2+ in porous media through their influence on relaxation times.359 There have not been as many studies of fluid flow in porous solids by NMR imaging as might have been expected considering its considerable industrial importance in processes such as oil recovery and ~ a t a l y s i s . ~ ’ ~ ~ ~ ~ This is caused in part by the large local magnetic field inhomogeneities caused by susceptibility differences between the solid and the liquid. This means that the effective transverse relaxation time, T;, is often very small. However, it is still possible to obtain rapid images using methods such as a modified echo-planar imaging technique.365 NMR imaging has also been used to attempt to resolve changes in pore size distributions during drying of materials made by a sol-gel process.3hh


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Recently, NMR imaging has successfully been applied to the study of water within alumina and silica catalyst support pellets, allowing both the pore structure and transport processes to be explored in the same experimental environment.367 Spin density, relaxation time and diffusion measurements within the catalyst support have been made. The results show that there is significant structural heterogeneity within the catalyst support, reflecting variations in pore size that arise during preparation of the support pellet, and these heterogeneities can be directly imaged.367 It is also now possible to combine NMR imaging with PFG NMR to obtain velocity and diffusion measurements on fluids in solid^.^^^^'^ For instance the adsorption and diffusion of n-hexane in a bed of zeolite Na-X has been monitored. It is found that intracrystalline diffusion is fast enough to ensure total adsorption of individual zeolite crystallites before the macroscopic adsorption front has moved significantly further into the bed.371 ACKNOWLEDGEMENT

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Miscibility, Morphology and Molecular Motion in Polymer Blends K. TAKEGOSHI Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan

1. Introduction 2. Miscibility 2.1. Spin-lattice relaxation experiments 2.2. Spin-diffusion experiments 2.3. Heteronuclear cross-relaxation experiments 2.4. Other experiments 2.5. Concluding remarks 3. Polymer-polymer interaction 3.1. Chemical shift 3.2. Nuclear Overhauser effect (NOE) 3.3. Molecular motion 4. Morphology 4.1. NMR imaging 4.2. Lineshape 4.3. Spin-lattice relaxation times and spin diffusion 4.4. Thermally induced morphological change 4.5. Microscopic heterogeneity 5. Molecular motion 5.1. Glass transition 5.2. Effect of blending on local motion 5.3. Motional heterogeneity Acknowledgement References

97 101 102 104 108 109 109 110 111 112 114 115 115 116 118 119 122 122 122 124 125 126 126

1. INTRODUCTION

To satisfy various engineering needs, polymer scientists have been trying to find new polymeric materials. A variety of new monomers and new polarization techniques have been exploited day by day. At the same time, chemical and physical modifications of existing polymers such as grafting, cross-linking, block-copolymerization, interpenetrating-network formation and blending have been widely examined. Much of the usefulness of these multicomponent materials derives from the inherent possibility to modify ANNUAL REPORTS O N NMR SPECIXOSCOPY VOLUME 30 ISBN 0-12-505330-4

Copyright 01995 Academic Press Limited AN rightr of reproduction in any form reserved

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K. TAKEGOSHI

macroscopic properties. Among these modifications, polymer blending provides a simple and economical means of combining useful properties of component polymers.' Whether a blend shows an average property of component polymers or keeps original properties of them depends on the degree of microscopic mixing and its phase structure.2 The degree of mixing has been described in terms of miscibility of molecular to macroscopic scales, domain size and phase structure, compositions of each domain, e t ~ . ~ Many instrumental methods have been applied to investigate structures of blend^.^ To name a few examples, differential scanning calorimetry (DSC) and dynamic mechanical spectroscopy (DMS) examine miscibility and motional behaviour. Scattering measurements (small angle X-rayheutron scattering (SAXS/SANS), light scattering and pulse-induced critical scattering (PICS)) concern the size and distribution of domains as well as the repeat period of a domain structure. Electron microscopy (for example, tunnelling electron microscopy (TEM) and scanning electron microscopy (SEM)) provides morphology of domains. Usually one method alone provides a limited view of a blend. The first application of solid-state nuclear magnetic resonance spectroscopy (NMR) to examine the microstructure of a blend is the work of Kwei and co-working on polystyrene/poly(vinyl methyl ether) (PS/PVME) .5 They measured 'H spin-lattice relaxation time ( Tl) and spin-spin relaxation time (T2) over a wide temperature range. The invention of high-resolution solid-state 13C NMR spectroscopy in 1976 by combining high-power proton decoupling and magic angle sample spinning (MAS)6 opened new possibilities to study solid materials. Nowadays, most polymer researchers employ high-resolution NMR in liquids and in solids to characterize what they obtain from reaction vessels. Not simply using NMR as an analytical tool, many researchers use NMR to provide a molecular interpretation of macroscopic properties of synthetic polymers in bulk.7 Several NMR techniques have been successfully applied to investigate phase structure, heterogeneity and molecular motion of pure polymers. Heterogeneity in a blend is much more easily appreciated than that in pure polymers, because each component polymer in a blend can be discriminated by its distinct peaks. This facilitates the study of the domain structures of the component polymers in a blend. The first high-resolution I3C NMR study on a blend is the work of Schaefer et al. on polystyrene/polystyrene-polybutadieneblock copolymer (PSIPS-PB)8 and PS/poly(2,6-dimethyl-l ,Cphenylene oxide) (PS/ PPO)' in 1981, and there are growing numbers of NMR studies of blends. In Table 1 are collected more than 100 NMR studies of polymer-polymer blends published in the literature up to the end of 1993. These are mainly collected from two journals, Macromolecules and Polymer, and one can see the increase of publications within these few years. In this review, the main discussion concerns I3C high-resolution solid-state NMR measurements applied to a blend of two polymers. Since miscibility is

MISCIBILITY, MORPHOLOGY A N D MOLECULAR MOTION

Table 1. Polymer pairs studied by NMR.'

Polymer I

Polymer I1

PAA PAC PAC PAEK PAr PB PB PB PBA PB I PB I PBMA PBT PBZMA PBZT PC PC PC PC PC PCHMA PChP PCL PCL PCL PDMA PDMA PDMA PDMA PDMA PDMS PDMS PDMS-PDPS PE PEA PEA-PMVPyl PEA-PMVPyI PECMA PEMA PEMAD PENDC PEO PEO PEO PES PES PET PET PET

PVA PS PE PEI PBT PIP PS SBR PVPh PEI PIm PS-PVPh PC PVC Nylon 66 PAN-PMA-PB PCHDMT PET PMMA PS PVC PIP PVC PVME PVPh PHMP PS PS-PSSA PS-PVPh PVPh Silicone PDMS-d Silicone PP PS PS-PSSA PS-PAAMA PNBEA PVF2 PAN-PMA-PB PENDC-PHBZA PMMA PVC PVPh PIm PPS PENDC PENDC-PHBZA Vectra

Reference

10 11 12 13 14, 15 16 17, 18 19 20 21, 22 23 24, 25 26 27 21 46 28 29, 30 28, 31 32 27 33 34 35a 20 36 37 37 37 37,38 39 40 39 41 42,43 43 43 44 45 46 47 48-50 51 52,53 54 55 47 47 56a

99

100

K. TAKEGOSHI

Table 1. (contd.) Polymer pairs studied by NMR.“

Polymer I

Polymer I1

Reference

PETA PETS PIP PIP PMA PMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA PMMA- PVPy POT PPO PPO PPO PS PS PS PS PS PS PS PS PS-d PS-PSSA PS-PSSA PVA PVE PVMK PU PU

PVPh PVPh epo-PIP PVE PVAc PVPh PVA PMMA-d PS PS-PAN PS-PVPh PVAc PVC PVF2 PVPh PU PS-PSSA PPO P2MS P4MS PS Nylon 6 PS-d PVIZ PVIZ-PEA PVME PVPy PVPy-PEA PU PS-PB Nylon 6 PU PVP PVE-d PVPh PU-d Lignin

20 20 33 57 58 59 10 60 61, 62, 67, 87 63,64 65 66 27, 67, 68 45,64,6972 59,73 74 61,75 76 77 77 9,56b, 78-81 82 8, 83 42 42 5, 35, 67, 84-92 42 42 93 8 94,95 96 97,98 57b 99 40 100

“A copolymer is hyphenated and a deuterated polymer is denoted by “-d”. Abbreviations: Nylon 6, poly(s-caprolactam); PAA, poly(acry1ic acid); PAAMA, poly(tetraalkylammonium methacrylate); PAC, polyacetylene; PAEK, poly(ary1 ether ketone); PAN, poly(acrilonitri1e); PAr, polyarylate; PB, polybutadiene; PBA, poly(buty1ene adipate); PBI, polybenzimidazole; PBMA, poly(buty1 methacrylate); PBT, poly(buty1ene terephthalate); PBZMA, poly(benzy1 methacrylate); PBZT, poly(benz[a,d]dithiazol-2-6-diy1-1,4-phenylene); PC. polycarbonate; PCHMA, poly(cyclohexy1 methacrylate); PCHDMT, poly(cyc1ohexylenedimethylene terephthalate); PChP, polychloroprene; PCL, poly(s-caprolactone); PDMA, poly(N,N-dimethylacrylamide); PDMS, poly(dimethylsi1oxane); PDPS, poly(diphenylsi1oxane); PE, polyethylene; PEA, poly(ethy1 acrylate); PECMA, poly([N-ethylcarbazoI-3-yl]methyl methaacrylate); PEI, poly(ether imide); PEMA, poly(ethy1 methacrylate); PEMAD, poly(ethy1ene-co-maleic anhydride); PENDC, poly(ethy1ene naphthalene dicarboxylate); PEO,

MISCIBILITY, MORPHOLOGY AND MOLECULAR MOTION

101

very important for a blend, various NMR techniques have been developed, which are discussed in Section 2. Section 3 takes up the questions of how to specify the origin of interpolymer interactions between component polymers. Without such interactions, most of a polymer pair does not mix well (immiscibility). In Section 4, NMR approaches to study heterogeneity in a blend are discussed. It also includes the interface of domains and phase separation. Finally, in Section 5, molecular motion in a blend is discussed. For some polymer pairs, microscopic mixing at the molecular level is achieved. Some DSC studies have shown that microscopic mixing leads to averaging of the two Tg values of component polymers. Does microscopic mixing force component polymers to move together? We will see that chain dynamics of component polymers affect each other. However, there has been no apparent NMR evidence of a correlated motion of two dissimilar polymer chains. 2. MISCIBILITY

Macroscopic properties of a blend are influenced largely by its microscopic degree of mixing. The degree of mixing (i.e. miscibility) varies from very homogeneous mixing, where the blend acts like a single-phase system, to very heterogeneous mixing, for which a separated domain of a component polymer is dispersed in a matrix of the other polymer. If the size of a domain is smaller than the characteristic space scale of a particular observation, the blend appears to be homogeneous. Various methods have been used to examine miscibility, for example, dielectric and dynamical relaxation, lightheutron scattering and infrared and NMR spectroscopies with the characteristic space scales. For instance, an observation of a single glass transition at a composition-dependent temperature has been taken to show evidence of miscibility. However, the smallest domain size that can be studied by glass transition measurements is ca. 100

poly(ethy1ene oxide); PES, poly(ether sulphone); PET, poly(ethy1ene terephthalate); PETA, poly(ethy1ene adipate); PETS, poly(ethy1ene succinate); PHBZA, poly(p-hydroxyhenzoic acid); PHMP, poly([l-hydroxyl-2,6-phenylene]methylene);PIm, polyimide; PIP, polyisoprene; epo-PIP, epoxidized PIP; PMA, poly(methy1 acrylate); PMAA, poly(methacry1ic acid); PMMA, poly(methy1 metacrylate); PMVPyI, poly(N-methyl-4-vinyl pyridinium iodide); PNBEA, poly(2-[3,5-dinitrobenzoyl)oxy]ethyl methacrylate); POT, poly(3-octylthiophene); PP, polypropylene; PPO, poly(2,6-dimethyl-l,4-phenyleneoxide); PPS, poly(pheny1ene sulphide); PS, polystyrene; PSSA, polystyrene sulphonated; PU, polyurethane; PVA, poly(viny1 alcohol); PVAc, poly(viny1 acetate); PVC, poly(viny1 chloride); PVE, poly(viny1 ethylene); P W , poly(viny1idene fluoride); PVIZ, poly(N-vinylimidazole); PVME, poly(viny1 methyl ether); PVMK, poly(viny1 methyl ketone); PVP, poly(N-vinyl-2-pyrrolidone); PVPh, poly(4vinylphenol); PVPy, poly(4-vinylpyridine); E M S , poly(2-methyl styrene); P4MS, poIy(4methyl styrene); SBR, styrene-butadiene rubbers; Vectra, poly(p-hydroxylhenzoic acid-co-phydroxylnaphthoic acid).

102

K. TAKEGOSHI

In most NMR methods discussed below, microheterogeneity manifests itself in the magnetic relaxation phenomena such as spin diffusion and spin-lattice relaxation. The relaxation phenomena in solids are mostly governed by a dipole-dipole interaction, which is a function of the distances between the spins concerned. In the following, we will see that in favourable cases, the spatial resolution of solid-state NMR reaches down to 20 A. 2.1. Spin-lattice relaxation experiments

Each 'H spin has its intrinsic spin-lattice relaxation rate. In solids, these different relaxation rates tend to be averaged by a mechanism called spin diffusion.' 0 2 Spin diffusion is the equilibration process of non-equilibrium polarizations of spin systems at each local site through mutual exchange of polarization. Since the efficiency of spin diffusion is governed by a strength of a dipoledipole interaction, the rate of spin diffusion among the 'H spins of component polymers in a blend would provide information about the domain size. In this section, the effects of spin diffusion on the spin-lattice relaxation process are discussed. The 'H spin-lattice relaxation is monitored via resolved 13C signals to enjoy higher resolution of 13C spectra. This enables us to observe the relaxation of a component polymer separately. Stejskal et al. studied the 'H spin-lattice relaxation process in the rotating frame of reference (Tl process) of PPOlPS via the 13C signal intensities of PPO and PS (Fig. 1).B Observed non-exponential decays were successfully explained by the following model. Firstly, the 'H spins are divided into two species, depending on the polymer they are part of (species A for PPO, species B for PS). One can simplify the problem by considering that both 'H spins A and B have their own relaxation rates K , and Kb, respectively. By assuming very fast spin diffusion among 'H spins within a same polymer chain, and moderate spin diffusion between A and B, the coupled equations for the decay may be derived as

+

-dAldt

=

(K,

-dBldt

=

(Kb+f,k,)B-fbk,A

fb

k,) A - f a k, B (1)

where k, is the spin-diffusion rate between A and B , and fa and fb are the ratios of the numbers of spins A and B, respectively. The observed decay is fitted satisfactorily by the values shown in the figure. k, can be related to the domain size and its structure, based on a proper model of morphology. lo3 From the 'H relaxation experiments, however, the value of k, can be obtained only when the characteristic non-exponential decay curves are observed. From numerical calculations of equation (l),one may note that only when k, is of the order of K , and Kb is such non-exponential behaviour appreciable. In many blends, k, is so fast that


103

10

PPO

PS (high MW) m m ( r ._

I

-

I n 1 m

_I 0

PPO i-PS

0.1

I

0

I

I

I

1

20

10 t, (SLL ms

Fig. 1. Observed T I , decay curves of PPO and PS in PPO/PS = 75/25 blends. The pair of calculated curves by using equation (1) with the fitting parameters indicated in the figure are shown as solid lines. (Reprinted with permission from ref. 9. Copyright 1981 American Chemical Society.)

the two decay curves from polymer A and polymer B are identical or k, is too slow to average the two decays. In these extreme cases, one cannot obtain the k, value. To estimate spatial information here, the diffusion equation is solved by assuming a proper model for a 'H spin system of a blend. For a heterogeneous polymer, one may assume lamellar morphology. Then a typical diffusion path length, x, for a short diffusion time in one dimension can be given as213104 x2 = 4Dtl3 (2) where D is the spin-diffusion constant. If one starts from a different model such as diffusion from a point the factor 4/3 in equation (1) may be different. Fortunately, however, such differences depending on the model chosen would not become serious in a deduced domain size. The

104

K. TAKEGOSHI

spatial resolution thus depends on the diffusion constant, D. Assink gave a simple relation between D and the spin-spin relaxation time, T2 asIo5

D = 2(r0)2/T2 (3) where ro is the proton van der Waals radius of 1.17 A. From this equation, the D value for T2 of 50 ps is 5.5 x cm2 s-'. For 'H spins in alkanes, D was calculated to be 6.2 X cm2 s-l;lo6 for PET, D was estimated to be 5X cm2 s - ' . ~By~ comparing the NMR results with SAXS and TEM data, Clauss et al. decided D for PSRMMA as 8 X cm2 s - ~ They . ~ ~ ~ postulated this value as a reference value for materials with pronounced mobility. All D values in the literature are in a range of cm2 to s-', and I adopt a value of 5 x cm2 s-'. For a diffusion time of 1s, the mean-square diffusive path length ( x ~ ) "is~ calculated to be ca. 200A. This means that if one observes the IH spin-lattice relaxation (TI) in the time scale of 1s, all 'H spins within 200 A appear to have an identical relaxation time. If we observe only one single T I of 1s for both component polymers and assume a diffusion time similar to the relaxation time,34'45,63,68 we can conclude that the domain size is smaller than 200 A.Since a typical Tlp value is of the order of milliseconds, the T I P experiment can be used to investigate the domain size on the scale of a few 10 A.Due to its facility, the 'H spin diffusion measured via the Tl and T1, relaxation times is frequently used to establish the length scale over which the blend is homogeneously mixed. Various factors affecting miscibility, such as molecular eight,^^,"^ side-chain d i f f e r e n ~ e , ~ number ~'~~,~ of~ , ~ ~ monomer units in the copolymer,25and ta~ticity,~' have been studied by the IH spin-lattice relaxation experiments. Also, many works examined composition dependence of miscibility. 2.2. Spin-diffusion experiments

Instead of monitoring spin-diffusion effects from relaxation, several experiments have been invoked to observe 'H spin diffusion directly. The spin-diffusion experiment consists of four periods: (1) the preparation of the non-equilibrium magnetization among the 'H spins of the component polymers or between different domains; (2) the variable spin-diffusion time, t, during which spin diffusion takes place;

(3) the observation of the resulting 'H magnetization; (4) the relaxation time during which the whole 'H spin system achieves the Boltzmann equilibrium. Note that these procedures are formally analogous to cross-relaxation and

MISCIBILITY. MORPHOLOGY AND MOLECULAR MOTION

105

chemical exchange NMR experiments in liquids. Various experimental methods for (1) and (3) are demonstrated. For (l), one may select 'H spins by their resonance frequencies. One can also use a T2 difference for mobile and rigid 'H spins. For (3), one may enjoy high sensitivity of 'H by observing 'H directly, or enjoy high resolution of 13C resonances. 2.2.1. Goldman-Shen spin-diffusion experiments One simple way to achieve non-equilibrium magnetizations for 'H spins in a heterogeneous system is to utilize mobility difference. Goldman and Shedo7 have developed an experiment that may be used to monitor spin diffusion between regions of a heterogeneous system described by significantly different spin-spin relaxation times (T2). The original Goldman-Shen experiment is modified to incorporate the high-resolution 13C d e t e c t i ~ n . ~ This ~ " ~ 'modified ~~ experiment is performed as follows. The free-induction decay (FID) of the 'H transverse magnetization of a heterogeneous system may be described by a sum of FIDs with different T2 values reflecting different mobilities of 'H atoms. After a certain delay time, a shorter T2 component of the 'H magnetizations vanishes, and only a longer T2 component remains. The remaining 'H magnetizations are flipped back to the z-direction by an r.f. pulse. For time t , the distribution of the magnetizations becomes gradually uniform through spin diffusion. The resulting 'H magnetizations at a time t are measured indirectly by transforming it to I3C by cross-polarization (CP). The longer T2 component may be attributed to one of the mobile component polymers or a side chain such as a fast reorienting methyl group. We are to observe spin diffusions from mobile to rigid parts. Figure 2 plots normalized deviations of 13C magnetizations, P(t) = ( M ( t )-Mo)/Mo, for PMA and PVPh versus the square of the spin-diffusion time,59c where M ( t ) is the magnetization after the diffusion time t and Mo is the magnetization at internal equilibrium. An effective spin-diffusion time te is defined as the intercept of the straight line with the abscissa,21 and the domain size can be calculated from equation (2). By this technique, miscibility of several blends has been studied: PEI/PBI and PBZT/nylon,21 PES/PP0,55 PS/PP0,79 PEO/PVPh,52a PMA/PVPh.59c There are further modifications made on the modified Goldman-Shen experiment. Instead of using the T2 difference, Zhang and Wang applied the spin-locking pulse during the delay time to select 'H spins by their TIP difference.55bClauss et al. applied the multiple-pulse dipolar filter during the delay time.62b The application of the multiple pulse renders the T2 difference and helps us to differentiate component polymers. Schmidt-Rohr et al. incorporated two-dimensional (2D) NMR techniques: the first dimension gives the 'H wideline spectrum and the second dimension the I3C high-resolution spectrum.92

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-0.4

1

-- 1.2 O . Y

Fig. 2. Plot of P(t) versus square root of spin-diffusion time f1'2 for PMA/PVPh = li 1.5 blends at 361 K. Symbols represent data observed through the methoxy carbon of PMA (a), the aromatic carbons of PVPh (V)and the main-chain carbons of both PMA and PVPh (A)in the blend. Straight lines are drawn through the linear portion of the data at an early stage to determine the intercept time te. (Reprinted with permission from ref. 59c. Copyright 1992 The Society of Polymer Science, Japan.)

2.2.2. Exchange N M R experiments Caravatti et al. observed 2D 'H exchange spectra of PS/PVMEs6= (Fig. 3). High-resolution 'H NMR is achieved by applying a multiple-pulse technique and MAS. The cross-peaks between different 'H species represent spin diffusion among them. For immiscible PS/PVME obtained from chloroform (Fig. 3(a)), only intrapolymer cross-peaks within PS or PVME are observed. For miscible PS/PVME from toluene, interpolymer cross-peaks indicated by arrows appear (Fig. 3(b)), showing direct evidence of spin diffusion. Since the 2D experiment is time consuming, they also postulated two onedimensional (1D) spin-diffusion experiments.86b By changing the spindiffusion time in the 1D experiments, they determined the composition of the mixed phase and spin-diffusion rate constants. VanderHart et al. applied similar 1D spin-diffusion experiments to study miscibility and stoichiometry in a blend.21,22*24 For many blends, the resolution of 'H spectra is not enough to discriminate component polymers. As for the modified Goldman-Shen sequence, Spiess et al. combined a high-resolution 13C NMR technique with


PS -CH-CH,I

PVME

OCH, OCH

-CH-CH,-

aliph. ti

107

0 I CH,

f\

Fig. 3. Two-dimensional proton spin-diffusion spectra of PS/PVME in solids: (a) cast from chloroform; (b) cast from toluene. The mixing time is 100ms. Interpolymer cross-peaks between aromatic protons on PS and the methine and methoxy protons on PVME are indicated by arrows. (Reprinted with permission from ref. 86a. Copyright 1985 American Chemical Society.)

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K. TAKEGOSHI

the 1D spin-diffusion experiment. 13,62b They selected 'H magnetizations of one component polymer based on 'H chemical shift or mobility. The resulting 'H magnetizations after variable spin diffusion times are transferred to 13C spins by CP and observed. This method enables us to enjoy higher resolution of I3C. Henrichs et al. applied the lD/2D '3C-'3C exchange NMR to study PC/PET.*' Miscibility and the spin-diffusion rate can be deduced from the interpolymer cross-peaks between well-resolved 13C resonances. Since for 13C both natural abundance and gyromagnetic ratio are small, 13C-13C spin diffusion is much less efficient than that among 'H. To facilitate experiments, Henrichs et al. used 13C enriched polymers. Since isotopic enrichment is time consuming and expensive, the rotational resonance technique'" was adopted to recover the l3C--l3C dipole interaction averaged by MAS.56This realizes 2D 13C-13C exchange spectra without enrichment of I3c.

2.3. Heteronuclear cross-relaxation experiments

So far, NMR techniques utilizing homonuclear dipole interactions have been reviewed. In this section, we summarize techniques to examine crossrelaxation phenomena induced by heteronuclear dipole interactions. One of the typical cross-relaxation phenomena is the transient nuclear Overhauser effect (NOE), which is a function of interspin distance. The NOE between 'H and "F spins was observed to study miscibility of PMMA/PVF2.45White and Mirau observed interpolymer NOE between protons of PVME and carbons of deuterated PS.35cThis technique may be applicable to a mobile polymer blend. A cross-relaxation phenomenon frequently used to study miscibility is interpolymer CP from protons in one component to carbons of another deuterated component.8'47'57b,61,67,87 Without interpolymer CP, the 13C signal intensity of a deuterated polymer should be much smaller than those of the protonated ones. Therefore, an appreciable signal enhancement for the deuterated component is taken as evidence of miscibility. Since effective 'H-13C CP transfer is limited to about 10 A, the signal enhancement of the latter carbons by CP shows that at least parts of the protonated molecules are nearby. For the interpolymer 'H-I3C CP experiment, deuteration of one of the component polymers is not a prerequisite. Grinsted and Koenig undertook close examination of the CP rate upon blending and ageing.72 The observed decrease of the CP rate with ageing was explained by phase separation. Zhang et al. also examined the interpolymer CP effects for PVA/PVP.97 The CP rate of the non-protonated carbonyl carbon of PVP increases upon blending with PVA, showing closeness of the OH proton of PVA and the carbonyl carbon of PVP.


109

For blends with unresolved I3C resonances, Zumbulyadis et al. showed that proton to deuterium CP transfer is useful to investigate mis~ibility.~',~~ The interpolymer CP experiment was performed for PMMNPVF2 in which 19Fpolarizations of PVF2 are transferred to carbons of PMMA.71 2.4. Other experiments

Walton et al. demonstrated the use of '29Xe NMR to study miscibility of blends. 16,33 It employs a density-proportional 129Xechemical shift. If '29Xe atoms are dissolved in domains of a blend and the domain size is so large that '29Xe atoms in different domains cannot exchange rapidly, a 129Xe NMR spectrum shows multiple resonances reflecting different environments around the Xe atoms. The authors found that an upper bound for the domain size studied is about 600 A. There are a few studies to appreciate effects of the additional 'H dipole field on spectra upon blending. Delayed or non-decoupled 13C NMR can be utilized to envisage whether 'H dipole interactions increase/decrease by blending.8,47,83For deuterated PS, VanderHart observed broadening of residual 'H spins in 'H MAS spectra when protonated PS is i n t r ~ d u c e d . ~ ~ The increased 'H dipole interactions would indicate that the protons of one component polymer come close to the other component upon blending.

2.5. Concluding remarks In this section, we have reviewed several NMR techniques to monitor spin diffusion. All techniques rely on a dipole-dipole interaction, which is a function of the internuclear distance. It is worth noting here that a dipole interaction is also a function of the angle between the static magnetic field and the internuclear vector. Therefore, modulation of a dipole interaction by molecular motion must be carefully considered in interpreting spindiffusion results. One manifestation of motion frequently encountered is the discrepancy of the calculated and the observed relaxation rates of a miscible blend. In Section 2.1, it was stated that a fast spin-diffusion averages unequal spin-lattice relaxation rates of component polymers. The apparent relaxation rate Raveis given as a weighted average of the rates of component polymers A and B as follows:45263 Rave

= f a Ra -/-

f b Rb

(4)

where f a and f b are the fractions of protons of the polymers A and B, respectively. In evaluating equation (4), one may use the relaxation rate of a pure polymer A for R , and that of B for Rb. Many authors found discrepancy between the calculated Rave and the observed Rave. As these

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K. TAKEGOSHI

authors pointed out, one cannot use R of the pure polymer, because blending sometimes changes molecular mobilities (Section 5). The averaging of Tg values upon blending also indicates alternations of molecular motion in the blend. MAS also modulates a dipole interaction. Haeberlen and Waugh reported that MAS has little effect on T I but has some effect on T2 and T1,."o TCkCly et al. reported that T2 of the mobile amorphous component becomes longer by MAS.'"8 Caravatti et al. noted that modulation of the dipole interaction by MAS becomes appreciable when the dipole interaction is partially averaged by fast molecular motion.86 Therefore, results of spin-diffusion experiments of mobile blends must be examined carefully. A temperature calibration is prerequisite to estimate the degree of molecular motion. From temperature calibrations using proton signals of methanol under spinning at 6 kHz, we found the temperature inside about 10°C higher than that for the driving/bearing gas of our spinning system. Both molecular motion and MAS may reduce spin diffusion, leading to overestimation of the domain sizes. Most of the above-mentioned techniques adopt the 13C detection in monitoring 'H spin diffusion. Despite the advantage of high resolution, one disadvantage may be anticipated. Although the spin-locking field suppresses spin diffusion partially,'" 'H spin diffusion occurs during the infinite CP period. This blurs heterogeneity and may lead to wrong conclusions. From our recent 2D 'H-13C heteronuclear correlation experiments' l 2 of PMA/ PVPh using CP, as for spin-diffusion/mixing, short CP times less than 200 ps may be sufficient to ignore the spin diffusion during CP.l13 However, such insufficient CP would lead to distorted 13C spectra.

3. POLYMER-POLYMER INTERACTION

According to modern thermodynamical theories of polymer b l e n d ~ , ' ' ~the Gibbs free energy of mixing is described by the following three contributions: (1) combinatorial entropy of mixing;

(2) free volume difference between the component polymers; (3) exchange interaction energy. Except for a few polymer pair^,^',^' the combinatorial entropy of mixing is too small to overcome the disadvantage of free volume difference and mixing is unfavourable for most polymer pairs. A certain exothermic interpolymer interaction should therefore be operative when dissimilar polymers are miscible. These interactions include charge transfer, dipoledipole, ion-dipole, ion-ion, acid donor-acceptor and hydrogen bonding. It


111

is of particular interest to investigate how and to what extent component polymers interact with each other. NMR spectroscopy provides information about such interpolymer interactions. In fact, most of the spin-diffusion experiments reviewed in Chapter 2 are potentially applicable to explore specific interactions. In this chapter, polymer interactions appreciated by chemical shifts, NOE and molecular motion are reviewed.

3.1. Chemical shift By comparing NMR spectra of component polymers with that of a blend, one can easily deduce any chemical-shift and/or lineshape change due to blending. The lineshape reflects morphology of a blend, and will be discussed in Section 4. Apparent chemical-shift change can be attributed to modifications of both chemical structure and polymer conformation upon blending, reflecting a specific interpolymer interaction. In solution, Djordjevic and Porter investigated the solvent-induced changes in chemical shifts of PPO/PS, and concluded that the driving force of miscibility of PPO and PS is the r-hydrogen bond between the electrodeficient methyl group in PPO and r-orbitals in PS.80 In solids, however, there have been no reports of apparent shifts in PPO/PS on blending, and the specific interaction was investigated by observing molecular motion (Section 3.3). In fact, many of the above-mentioned interpolymer interactions do not cause appreciable change in chemical shifts in solids. For example, Natansohn and Simmons examined 13C spectra of PECMA/PNBEA.44aModels for the structural units of the polymers show upfield shifts of 1-5ppm due to charge-transfer interaction. For polymer blends, however, the shift is less obvious due to a general broadening of signals. Grobelny et al. observed changes of the imide carbonyl lineshapes, which were attributed to increased polarity due to solvation of PIm by the PES chain.54 So far, only the hydrogen-bonding interaction is found to cause an apparent shift. Several blends show downfield shifts and changes of lineshapes due to hydrogen bonding. For example, Yang et al. found a broadening and a downfield shift of the carbonyl carbon resonance of PHMP in PHMP/ PDMA.36 Also Grobelny et al. found that blending PBI and PIm induces a broadening and a downfield shift of the aromatic phthalimide carbonyl resonance with respect to that of the pure material.23 This shift is interpreted as a result of the specific hydrogen bonding between the PBI imidazole amine group and the PIm carbonyl group. Kwei et al. closely examined the lineshapes of nylon 6/zinc salt of PS-PSSA to figure out the specific interpolymer intera~tion.’~ They found that the Zn2+ cation of the ionomer forms a complex with the amide nitrogen of nylon 6 and not with the carbonyl oxygen. Since the chemical shift of the phenolic carbon of PVPh is sensitive to

112

K. TAKEGOSHI

near-neighbour segment interactions, Belfiore et al.20~533yy and other via chemical shift/ g r ~ ~ pinvestigated ~ ~ ~ blends , ~ of ~ PVPh , ~ extensively ~ ~ lineshape changes. In these blends, PVPh is used as the proton donor and often a polymer having a carbonyl group as a side chain is used as a proton acceptor. Evidence for specific interaction is provided by a downfield shift of -3 ppm in the phenolic carbon resonance and shifthneshape changes of the carbons in the proton acceptor. Belfiore et al. examined the lineshape of the carbonyl carbons of several polymers in PVPh blends, and showed that the carbonyl lineshape is useful as a qualitative diagnostic probe of the polyester component's morphology and molecular mobility in partially miscible blends with PVPh.*' Another interesting proton donor is PVA. 10b,y7,98 The signal of the CHOH carbon of pure PVA shows the characteristic triplet lineshape due to hydrogen bonding. On blending with a proton acceptor such as PAA, rearrangement of hydrogen bonding occurs and the characteristic lineshape disappears. Such rearrangement also occurs in PAA and causes a shift for the carboxyl carbon. This clearly shows the formation of specific hydrogen bonding between the PVA hydroxyl group and the PAA carboxyl group. Contrary to the widely accepted statements that the hydrogen bonding brings a downfield shift, an upfield shift of about 3ppm is found for the carboxyl carbon of PMAA upon blending with PVA. lob The dissociation of the intrapolymer hydrogen bonding brings an upfield shift and the formation of blend causes a downfield shift. The observed highfield shift shows that the intrapolymer hydrogen bonding has a larger downfield shift effect than the interpolymer hydrogen bonding.

3.2. Nuclear Overhauser effect (NOE) Although the solid-state morphology is lost in solution, the specific interaction responsible for mixing in the solid state will be retained. The higher resolution in solution NMR enables us to investigate a specific region of interaction. In solution, dipoledipole interactions give rise to transient NOE between 'H spins. Since the strength of a dipole-dipole interaction is proportional to the inverse sixth power of the interproton distance, there occurs no appreciable NOE between protons separated by more than 5 A. NOE is, therefore, a sensitive probe to investigate whether component polymers are in close contact on a microscopic scale. Application of 2D NOE spectroscopy may reveal the interacting regions as interpolymer cross-peaks. Natansohn and E i ~ e n b e r gcarried ~~ out one- and two-dimensional (2D) NOE experiments and showed that the ionic interactions are operative between PMMA-PVPy and partially sulphonated PS (PS-PSSA). They found interpolymer NOE between the aromatic protons of PS and the methoxy protons of PMMA. Crowther et ~ 1 and. Mirau ~ ~ et ~ 1 . ~applied '

113

MISCIBILITY, MORPHOLOGY AND MOLECULAR MOTION PYME

PS m , ~0 0.00

! PSIPYME

2.0a

I*

$

CH

PYME

4.00

6.00

PS

1

13

P

8.00 I

I

I

I

I

8.00

6.00

4.00 PPM

2.00

0.00

Fig. 4. Two-dimensional phase-sensitive NOESY spectrum of PS/PVME (60 wt%) in toluene. The mixing time is 200 ms. Interpolymer cross-peaks between aromatic protons on PS and the methine and methoxy protons on PVME are circled. (Reprinted with permission from ref. 88. Copyright 1988 American Chemical Society.)

NOE experiments t o PS/PVME (Fig. 4). It was pointed out that interpolymer NOEs become appreciable only when the polymer concentration is more than 25 wt%. Above 40 wt% the interpolymer NOE between the aromatic protons and the methoxy protons of PVME is of the same magnitude as intrapolymer NOEs, indicating a specific i n t e r a ~ t i o n . ~ ~ However, for PCL/PVME, a polymer-polymer interaction is less specific. Similar NOE studies may be found in refs 10a, 37, 58, 59b, 69, 75. There are two factors that make it difficult to specify the interacting regions. One is the intrapolymer spin diffusion that redistributes magnetizations among protons nearby. If the spin diffusion rate is slower than the NOE rate, the initial build-up rate reflects only the NOE. In 1D N OE

114

K. TAKEGOSHI

experiments of PMNPVAc, Takegoshi et ~ 1 . ~found ' that the NOE between the methine protons of PMA and PVAc becomes appreciable only when irradiation times are longer than 500ms. The lag period indicated that the NOE peak of the former is caused by intrapolymer spin diffusion from the methoxy protons of PMA. The other factor may be the forcing contact of polymers at higher polymer concentrations. The increase in concentration reduces non-specifically the average interpolymer distance. Zhang el al. examined concentration dependence of the NOE value for PMA/PVPh.59b At higher concentrations than 40 wt%, the interpolymer NOE value between the OCH3 protons of PMA and the O H of PVPh is similar to that between the OCH3 of PMA and the phenolic ring protons of PVPh. Thus, it is difficult to specify interacting regions. However, they found that the former NOE does not have significant concentration dependence and below 20wt% only the NOE of the OH proton of PVPh is appreciable. This clearly indicates that PMA and PVPh is bound by hydrogen bonding. Concentration also influences mobility of polymers, and effects of molecular weight and concentrations are examined.35b The lack of intermolecular NOEs does not prove that the polymers are not in proximity, because NOE depends not only on a distance but also on the correlation time of motion of the interproton vector. If the correlation time of the motion is close to the inverse of the proton Larmor frequency, NOE is very weak and may not cause appreciable NOE cross-peaks. One may conduct experiments at different temperatures to shorten or lengthen the correlation time. Alternatively, the rotating frame Overhauser experiment (ROESY) may be helpful. As suggested by Kelts et ~ l . , ~ ROESY ' is also useful to differentiate cross-peaks from spin diffusion and NOE. It is worth noting here that the appearance of intermolecular NOEs does not guarantee homogeneous mixing in solutions. Zhang et al. showed that even in a phase-separated solution, NOEs between PMA and PVPh appear.59b The interpolymer NOE only indicates that a miscible phase exists, but not that the whole system is miscible. In solids, White and Mirau studied interpolymer NOE between the 'H spins of PVME and the 13C spins of deuterated PS to conclude that the phenyl ring of PS is much closer to the methoxy group of PVME than to the main-chain carbons of PS.35cIn fact, most of the spin-diffusion experiments discussed in Section 2 are potentially applicable to explore specifically interacting regions. However, the fast spin diffusion in solids may blur the specific regions.

3.3. Molecular motion

If there is a specific interpolymer interaction between component polymers, molecular motion of the interacting region of the individual polymer in the


115

mixture is expected to be different from that of pure solution. de Araujo et af. examined a possible specific interaction between the ether linkage of PPO and the phenyl group of PS by observing deuterium('H) spectra of the deuterated phenyl group of P S g l Such a specific interaction should reduce the fraction of mobile phenyl groups as compared with pure PS, due to less space available for conformational rearrangements. From experimental results, they concluded that such a specific interaction is not appreciable. On the other hand, Feng et al. found that after mixing, the aromatic carbons of PPO and PS come to have almost the same 13C spin-lattice relaxation times.79 They concluded that due to the strong r-7r conjugation interaction between the aromatic rings of PPO and PS, they move cooperatively. Bovey et af. have studied effects of blending on molecular motion in PMMA/PVF2 by 13C-T1 experiments in solutions.69 Both 13C-T1 values for pure polymers and for a mixture are the same. They concluded that the compatibility of these polymers, at least in solution, does not arise from complex formation between them. Comparison of the temperature dependence of l3C--T1 values has been made by Asano et al. for PC/PMMA.31a Compared to those of pure polymers, only the Tl-temperature curves of phenyl carbons of PC and that of the methoxy carbon of PMMA in the mixture shift toward high temperatures. The 13C-T1 curves of other carbons are not influenced by mixing. These results indicate that there is a specific interaction between the phenyl group of PC and the methoxy group of PMMA for PC/PMMA in solutions. Takegoshi et al. also applied 13C-T1 measurements to P M A / P V A C ~and ~ the effects of blending on TI curves are non-specific. 4. MORPHOLOGY

4.1. NMR imaging

The most comprehensive information about the morphology of a blend has come from microscope studies. For instance, the domain size and shape of the order ranging from 100 to 10 000 A can be visualized by TEM. Similar visualization may be possible by NMR imaging. In fact, NMR imaging has been a valuable tool in medical science to determine human morphology by locating water. In solids, the additional line-broadening due to the dipole interaction and the chemical shift interaction blurs the NMR image of the material. Therefore, the techniques used in medical imaging may not directly be applied to solid materials, and certain line-narrowing techniques such as multiple-pulse and MAS or large field gradient should be applied. However, for a mobile component whose 'H resonances are already narrowed by motion, extensive use of sophisticated line-narrowing techniques may not be necessary.

116

K. TAKEGOSHI

Cory et al. employed MAS for line narrowing of PB in PB/PS." The spinning speed of 5 kHz was enough to reduce the 'H linewidth of the mobile PB, and the NMR image of PB was obtained with spatial resolution of 50 pm. To realize imaging under MAS, one has to spin the field gradient synchronously with the sample. Sarkar and Komoroski applied conventional NMR imaging techniques to obtain images of elastomeric components of several tyre sections with resolution of 100-200 pm.I9 By applying strong (20 G cm-'), actively shielded gradients capable of fast switching (50100 ps), they could obtain images of 'H spins with T2 of a few milliseconds. 4.2. Lineshape

There are a few polymers that show different lineshapes for different morphology. One such polymer is nylon 6 , and it shows different lineshapes for the a- and y-crystalline and mesomorphous phases. By simulating the 13C spectrum of PShylon 6 from the three lineshapes, Schmidt et al. determined the fractions of the nylon 6 forms in the blend.82 Alternations in morphology of component polymers may occur on blending. For example, suppose we mix a crystalline polymer with an amorphous one. The blend may be miscible, that is, the crystalline phase is destroyed, and the whole blend becomes amorphous. Or the blend may be composed of a miscible amorphous phase and a crystalline phase. Such blending-induced alternations in morphology would be reflected in chemical shifts, linewidth and relaxation behaviour. Huo and Cebe found that for the melt-crystallized PBT/PAr, the NMR linewidth decreases as the PAr content increases, indicating more perfect crystals appear in the blends compared with that in pure PBT." They attributed this to the much slower crystallization of PBT in the blends. Belfiore et al. showed that the lineshape of the carbonyl 13C resonance reflects the polyester component's morphology and mobility in partially miscible blends with PVPh.*' The carbonyl signal in the crystalline domains exhibits a full width at half height of 1-2ppm when Tg is below the temperature of the NMR experiment. For a PVPh-rich blend in which no crystallization of the polyester component occurs and Tg is above the temperature of the NMR experiment, the linewidth increases to ca. 5-6 ppm. When the blends are completely amorphous, the carbonyl lineshape reveals at least two morphologically different microenvironments. Dumais et al. analysed solid-state 2H NMR spectra of deuterated PAC in PAc/PS to estimate the fraction of crystallinity." Molecular motion of PAC in PS is similar to pure PAC, indicating that domain structures are present in the blend. When the morphological difference does not cause an apparent shift, one still can reflect morphology from spectra by using the difference of relaxation times in different domains. Zhang et al. examined the T I , process

MISCIBILITY, MORPHOLOGY A N D MOLECULAR MOTION

I 90°*Y

117

DEC

9oox

PVPh/ P€0

Fig. 5. (a) Pulse sequence for selective measurement of a crystalline domain of PEO, and the resulting 13C CP/MAS spectra for (b) PVPNPEO = 58/42 and (c) 40/60. The signal from the crystalline PEO is indicated by an arrow. (Reprinted with permission from ref. 52a. Copyright 1992 American Chemical Society.)

in PEO/PVPh, and concluded partial miscibility for an excess of crystalline PE0.52a The blend consisted of the amorphous and crystalline phases of pure PEO and the miscible PEO/PVPh phase. The existence of crystalline PEO was confirmed by 13C spectra as follows. Since the 13C spin-lattice relaxation time of crystalline PEO (cu. 15 s) is much longer than those of amorphous phases (ca 0.1s), it is possible to observe the 13C spectrum of crystalline PEO selectively (Fig. 5(a)). Figure 5(c) shows the 13C-T1 selected spectrum of PVPh/PEO = 40/60, in which the signal of crystalline PEO is indicated by an arrow. On the other hand, for the PVPh-rich blend (PVPh/PEO = 58/42), the crystalline-PEO peak is not appreciable (Fig. 5(b)). There are reports describing the observation of 13Csignals of different domains separately using the variations of the dipolar field between the domain^.^,^' Due to the low sensitivity of NMR, observation of interfacial regions of a blend has been a formidable task. Yet, two groups observed the interface selectively with an enhanced signal-to-noise ratio. 12z3* Both groups employed highly polarized electron spins in one component polymer. The electron polarization is transferred to the 'H spins of the other component polymer at the interface region. The enhanced 'H polarization can be

118

K. TAKEGOSHI

observed directly12 or further transferred to the 13C spins for better r e s ~ l u t i o n .The ~ ~ electron spin was introduced by doping a radicalcontaining molecule to one component polymer,32 or the component polymer itself carried the unpaired electrons. l2 Even though the signal enhancement achieved is much less than that expected from simple theory ,12 Afeworki et af. observed the aromatic carbon signals of PC at the interface of It was also shown that the interfacial PC chains have less motion than PC chains in the To avoid leakage of polarization from the interface by the fast 'H spin diffusion, Afeworki attempted to transfer the electron polarization in the PS domain directly to the 13C spins of PC."'

4.3. Spin-lattice relaxation times and spin diffusion

Several studies have been done on immiscible blends to figure out the phase structure. McBrierty measured the temperature variation of 'H T I of PE/PP, which is i m m i ~ c i b l e The . ~ ~ observed increase of the TI minimum for the methyl group of PP and the concomitant slight decrease of the T I for PE was attributed to a weak PE-PP coupling via spin diffusion among two domains of component polymers. From T I and TI P results, Natansohn et af. concluded that PU/lignin is immiscible. However, they also found some interactions between two phases, which allow the interpolymer CP transfer from lignin to PU.'O0 A n immiscible blend of two dissimilar polymers A and B consists of three phases: two domains of polymer A and B and the interface. The 'H relaxation decay curves of an immiscible blend containing the three-domain structure has been examined by a direct 'H observation. For PB/PS, Segre et af. observed that the 'H FID consists of a fast- and slow-decaying component." The fast-decaying component was attributed to rigid PS, whereas the slow-decaying component shows the presence of two TI relaxations. These were attributed to the interface and to pure rubbery PB. For PU in a PMMA network, Parizel et af. observed that the 'H FID consists of three components, which were attributed to rigid PMMA, the intermediate region and mobile PU.74 In increasing temperature, they observed that the amounts of the latter two regions increase. For PMA/PVAc, the 'H T1 and T I , relaxation curves were analysed by a three-component superposition, and compositions of each domain were evaluated for P M N P V A C . Percec ~~ and Hammond observed the 'H TIP decay curves of component polymers of PUPAN-PMA-PB through the resolved I3C signals.46 The 'H T I , decay shows doubleexponential character, and the two TIPdecays of component polymers have one TIPcomponent in common. The common TIPcomponent was attributed to the interface. A blend of a crystalline polymer and an amorphous polymer tends to be immiscible when the crystalline component is excessive. The immiscible 18366774


119

crystalline-rich amorphous/crystalline blend consists of the miscible amorphous phase and the crystalline phase of one component polymer. TCkCly et al. observed a double-exponential decay for PMMA carbon resonances in PMMA/PVF2 when PVF2 is in excess. This is attributed to partial crystallization of PVF2.” Zhang et al. observed similar double-exponential T I or T1, decay curves for the crystalline component and a single exponential decay for the amorphous one.’0b,52b,55b,97 The relaxation time for the amorphous component is in agreement with one of the crystalline components, showing the existence of the miscible phase. By analysing the curves, the phase composition of PVA/PVP was evaluated.97

4.4. Thermally induced morphological change Thermodynamically, a polymer blend is rather unstable. Thus, its thermal history is an important factor in determining miscibility. In other words, heat-treatment is a simple technique to modify a phase structure of a blend. 4.4.1. Lineshape By annealing a crystalline/amorphous blend, one would expect a crystalline component polymer to recrystallize. The characteristic lineshape/resonance of a morphology can be used to monitor thermally induced morphological changes. By monitoring a peak characteristic of the a-crystalline phase of nylon 6, Gao et al. found that thermal annealing reproduces the crystalline phase in nylon 6/PS-PSSA.94 The methylene carbon resonance of PET in the trans conformation is narrower than that in the gauche form and appears at a higher field. By monitoring these resonances, Tang et al. found that the relative amount of the gauche conformer in PET/vectra is reduced greatly upon annealing, and its linewidth becomes Some studies use lineshapes to examine phase separation. At higher temperatures, the free-volume difference becomes too large to overcome the hydrogen-bonding interaction, and the phase separation occurs. Since the amount of the downfield shift is roughly proportional to the strength of the hydrogen bonding, it is possible to monitor phase separation of a blend by observing the spectra. Zhang et al. showed that when heating a PVA/PAA blend above its T g , the chemical shift of the carboxyl carbon of PAA shows a high-field shift. This high-field shift is attributed to dissociation of hydrogen bonding between PVA and PAA.”“ The 129XeNMR was applied to monitor the phase-separation process of PB/PIP. l 6 For a phase-separated, two-component blend, the 12’Xe NMR spectrum exhibits two resonances, whereas the homogeneous morphology of a miscible blend leads to a single peak. Although reactionldegradation is not a morphological change, I would

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like to include the works on these topics here. By examining l3C spectra of PC/PBT after heating at 270°C, Velden et al. concluded that transesterification occurred in a blend.26 Similarly, for PAr/PBT14 and PC/PET,30 transesterification at high temperatures has been found by 'H NMR in the solution state. These works may indicate a suitable arrangement for interpolymer reaction of dissimilar polymers in a blend. By examining TI values for annealed PC/PET, Henrichs et al. showed that, although PC and PET are not inherently miscible, at least at 2 W C , miscibility is induced by chemical reaction between polymers.29b With the hope of observing transesterification between PVA and PAA, Zhang et al. undertook heat treatment of PVA/PAA.lO" They observed dehydration of PAA. The products are similar to those from homopolymers, but the reaction temperatures for the blends are lower than those for homopolymers. 4.4.2. Relaxation behaviour The relative proportion of the phases in a blend may be reflected in relaxation decay curves. Annealinglageing of PMMA/PVF2 has been studied extensively. Grinsted and Koenig observed an increase of T l p and the cross-relaxation time between 'H and 13C with ageing of PMMMPVF2, indicating a subtle separation in the amorphous phase.72 By analysing the double exponential Tlp decays of PMMA/PVF2, TCktly et al. determined the degree of crystallinity as a function of annealing d ~ r a t i o n . ~From ' the Tl, values, they suggested that the crystalline phase is mainly the build-up of nuclei and lamellae of small dimensions. Papavoine et al. applied a triple-resonance 1H-13C-'9F CP technique to PMMA/PVF2.71dThey compared the crystalline fraction obtained by DSC and the isolated fraction of PVF2 measured by NMR as a function of the annealing time. They observed different crystallization behaviour depending on annealing temperatures, and suggested an upper critical solution temperature (UCST) for PMMA/ PVF2. Heating of PET/PC above the melting point of PET, followed by slow cooling, produced crystalline PET, as was indicated by the longer T I value for the PET proton after heating and by appearance of a slowly relaxing component of Segregation of domains in PMMA/PVAc after annealing at temperatures higher than Tg of PVAc was detected by analysis of T1,. 66 As shown above, stoichiometries of phases in an immiscible blend may be deduced by analysing multi-exponential relaxation decay curves. This approach was applied to study a phase-separation process. After spinodal phase separation, a two-mixed-phase morphology is assumed. Namely, two phases consist of both component polymers (A and B), but their stoichiometries are different. By assuming a fast spin diffusion within each phase, and a negligible spin diffusion between the two phases, one expects a double exponential decay for the 'H spins of polymer A. The fraction of the


121

two exponential functions should reflect the stoichiometry of the two separated phases. The decay of the 'H of polymer B is also a double exponential. Nishi et al. observed that a single exponential T I decay of PS/PVME becomes double exponential when annealed at 130"C.5.s4 The fraction of the two relaxations does change gradually with a phaseseparation period, leading them to conclude a spinodal decomposition at 130°C. Since they observed 'H directly, it was difficult to analyse fully the observed 'H decays by using the two-mixed-phase model. This has been overcome by extending the approach using I3C techniques. Asano et al. showed that stoichiometries of both major and minor components in a phase-separated domain can be obtained by analysing two 'H T I decay curves, selectively observed for component polymers via 13C.31b They determined the compositional change during a spinodal decomposition of PC/PMMA during the first 15 min (Fig. 6). Further heat-treatment does not cause any appreciable change in T1 curves. This shows that after a completion of the initial fluctuation in concentration, the morphological change occurs on a scale of more than 200-300 A, which does not affect the TI decays. A plot of the coexistent composition at each heat-treatment temperature versus the temperature gives us a phase diagram viewed from T I or T l p . Such phase diagrams are presented for PS/PVMEs9 and PC/PMMA.3'b VanderHart et al. applied a direct 'H observation using multiple-pulse methods and MAS to monitor spin diffusion in annealed PEI/PBI. From the spin diffusion results, they estimated the minimum Sinale Phase

-

0.50

J

-

Phase Separated 10190

+

0.51

1

+

Spinodal Decomposition

+-Q.484

0.52

5

3 1 8

\

-0.564

10

Fig. 6. Schematic illustration of the compositional change during heat-treatment of PC/PMMA = 50/50. The composition of PC is indicated by the underlined numbers. (Reprinted with permission from ref. 31b. Copyright 1992 The Society of Polymer Science, Japan.)

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domain dimensions in the annealed blends that display phase separation.22b Detailed discussions regarding the SAXS and NMR results are given. 4.5. Microscopic heterogeneity

Even for a miscible blend, conformational and packing heterogeneity would bring deviations of the isotropic chemical shift and local dynamics. Grobelny et al. studied miscibility of PBI/PIm by measuring T1p.23The TIPvalues for component polymers are similar; thus the blend is homogeneous as far as spin diffusion is concerned. The carbonyl carbon resonance, however, exhibits broadening. This broadening was attributed to different hydrogenbonding strength and distances in a blend. The lineshape was decomposed by assuming two carbonyl groups. The downfield resonance was attributed to the carbonyl group forming hydrogen bonding with PBI and the highfield one to the free PIm. Similar distribution of the resonance by different hydrogen bonding in a blend was found for the phenolic carbon of PVPh in PVMWPVPh99 and PEO/PVPh.53 The works exploiting microheterogeneity of local dynamics will be treated in the next section.

5. MOLECULAR MOTION Macroscopic properties of a blend, such as impact strength and ductility, are influenced largely by molecular motion of component polymers. Therefore, it is of importance to characterize motions in a blend, especially because blending affects molecular motions. In several blends, such effects of blending are appreciable in the relaxation phenomena. If both component polymers have the same Tlp value, it indicates miscibility. The observed T1, value for the blend can be calculated from the intrinsic T I P values for component polymers by using equation (4). Since it is difficult to measure the intrinsic T I , values for component polymers in the blend, one uses the T I , values for pure polymers. For several miscible blends, the calculated T1, values show apparent deviations from those expected from equation (4), showing that the TIPrelaxation values for respective component polymers are altered by blending. The effects of blending on motion may be caused from the different free volume of the blend compared with that of pure polymers, or specific interpolymer interactions between component polymers. 5.1. Glass transition

For an immiscible blend, one would expect that the component polymers keep their original mobilities, while for a miscible blend, entanglement on a


123

segmental scale would affect the motion of component polymers. Largescale chain motion in particular may be prone to being affected. In fact, the traditional mechanical and dielectrical experiments show that large-scale segmental motion of component polymers, which are related to the glass transition, are affected by blending. Moreover, for a miscible blend, a composition-dependent glass transition occurs at a temperature between the two Tg values of the respective component polymers. In NMR, the onset of large-scale chain motion at the glass transition affects 'H lineshapes. For PVME/PS = 50/50, Kwei et al. observed a transition of 'H T2 from 15 ps to 3 ms at ca. 50"C, which was ascribed to the onset of large-scale motion at the glass t r a n ~ i t i o n .The ~ FIID is composed of two T2 components, and each shows a different transition temperature. They are different from those observed for pure PS and PVME, showing effects of blending on large-scale motion, responsible for the glass transition. For PEO/PMMA, Brosseau et al. found that the temperature dependence of T2 of PE O obeys a Williams-Landel-Ferry equation with a temperature reference shifted 50 K higher than Tg.'' The molecular weight dependence was also studied. T o observe PEO in PEO/PMMA selectively, they deuterated PMMA. The transition of 'H T2 or the change in linewidth occurs when the frequency of motion exceeds the 'H linewidth, so that an averaging of the 'H-'H dipole interactions becomes appreciable. Similar motional narrowing occurs for a dilute spin such as 13C and 29Si. The linewidth of a dilute spin is governed by the anisotropic chemical shift interaction and the heteronuclear dipole interaction between 'H. The linewidth of a13C spin is typically a few 10 kHz for a rigid solid, therefore, the linewidth of a dilute spin is also sensitive to motion of a few 10 kHz. Newmark and Copley measured *'Si NMR spectra of silicone in P D M S / ~ i l i c o n e .The ~ ~ linewidth decreases gradually as the temperature is increased beyond Tg from 1200Hz at -100°C to 450 Hz at 150°C. The linewidths have been correlated with Tg and rheological data. For high-resolution solid-state 13C NMR using 'H dipolar decoupling (DD) and MAS, however, the heteronuclear dipole interaction between 'H and the anisotropic chemical shift interaction are effectively averaged. Thus, effects of motion on linewidth are rather complicated. Instead of motional narrowing, motional broadening is observed for a 13C spin under D D and MAS.'" It is because random molecular motion interferes with the artificial coherent averaging ( DD and MAS). When the motional frequency is close to the MAS frequency, the chemical shift anisotropy is reintroduced. While the heteronuclear dipole interaction is reintroduced, when the motional frequency is close to the 'H decoupling field strength in hertz. With a conventional static magnetic field of ca. lOT the former is negligible for anisotropic motion. Therefore, the observed broadening of 13C lines above the glass transition temperature has been attributed to the interference between motion and DD. For most glassy polymers below T g , the linewidth of a 13C resonance is

124

K. TAKEGOSHI

temperature independent and is about 2-3 ppm. In increasing temperature over T,, the line broadens, and reaches a maximum width. Further increase of temperature narrows the line to a few 10Hz. This behaviour was explained as follows:9o below Tg, the linewidth reflects a distribution of the isotropic chemical shifts arising from a variety of local conformations of the polymer in the glassy state. It is averaged by the motion associated with the glass transition to show motional narrowing. On the other hand, the interference occurs when the motional frequency comes close to the decoupling frequency, and the line shows maximum broadening. At much higher temperature, motional narrowing becomes effective, and the line becomes narrower even without D D and MAS. Takegoshi and Hikichi gave a quantitative equation to analyse the temperature dependence, and applied it to the C H carbon of PVME in PS/PVME.90 Menestrel et al. investigated the 13C resonance of PS in PS/PVME.91 Miller et al. observed temperature dependence of the 13C linewidths for PIP/PVE.57a The maximum linewidths for pure PVE and pure PIP occur at 302 and 250K, respectively. The difference is 52 K for pure polymers, while that for the blend becomes about 25 K, even though the blend exhibits a single Tg. Similar behaviour has been found for PEO/PVPh52aand PMA/PVPh.59aThese studies show that despite the thermodynamical homogeneity, the component polymers exhibit distinct glass transitions. The large-scale motions associated with the glass transition of the two component polymers affect each other appreciably. However, the chain dynamics of the two polymers are not correlated and still have different characteristic temperature dependences.

5.2. Effect of blending on local motion The lineshape studies described above concern motions of a few tens of kilohertz. There are several works exploiting effects of blending on local motions in the megahertz range by measuring the spin-lattice relaxation times of 13C. Feng et al. measured I3C-T1 for PPO/PS.79 The l3C-TI values for the aromatic carbons of PPO and PS become similar for a blend, suggesting that the aromatic rings of PPO and PS move with the same frequency. An analysis of the observed 13C-T1values led them to conclude that the blending increases the motional frequency of PS nearly 10 times. In addition to this, changes of mobility of PS were related to the improved impact strength. For a partially miscible POT/PPO, Schantz and Ljungqvist found that I3C-T1values of alkyl side-chain carbons increase considerably in blends of POT concentrations less than 30%, reflecting an increased flexibility for POT on blending.76 They suggested that the increased mobility is caused by thermal stresses in the material. Landry and Henrichs applied dynamic mechanical spectroscopy and 2H NMR to investigate sub-T, motion in PC/PMMA and PC/PCHDMT.28 Examination of *H NMR spectra and


125

relaxation times led them to conclude that local motions in the PC backbone are slower in the miscible blends than they are in pure PC, while local motions of PMMA are relatively unaffected by blending.

5.3. Motional heterogeneity As discussed in Section 4.5, conformational and packing heterogeneity exist in a miscible blend. This heterogeneity would manifest itself as a distribution of correlation times or motional frequencies. It is worth while to note here that there is another type of distribution of correlation time T . To treat the spin-lattice relaxation of anisotropic motions of a neat polymer, the correlation function of the orientation of an internuclear vector, Y(t), may be given as116

1'

Y ( t + T ) Y ( t ) = I Y ( t ) - Y ( t ) l G(T)exp(-dt)dt

(5)

where G(T)is the distribution function. Note that this distribution does not mean that there are several spins having different T , and the 13C spin-lattice relaxation of the whole spin system is described by a single exponential. This approach may be applicable to appreciate motions of polymer chains in a blend without defining a clear motional mode. It is difficult to differentiate the origin of the distribution in polymer materials. Jones et al. studied glass transition dynamics of PPO/PS, which is miscible.78 Chain motion of the I3C labelled PPO was investigated by I3C 2D exchange powder patterns.78b Further, chain motion of the deuterated PS was studied by 2H exchange powder patterns.78cThe motion of PPO of a few kilohertz commenced at a temperature of ca. 10°C below Tg,in contrast to the pure PPO glasses which only show such motion at temperatures above Tg. Both motions exhibited the characteristics of rotational Brownian diffusion with an associated broad distribution of correlation times. The distribution is a bimodal distribution and considerably broader than that typical for the pure polymer. They employed a statistical lattice model to evaluate local concentration fluctuations and explained the observed relative ratio of the modes. Schmidt-Rohr et al. applied the 2D version of the extended Goldman-Shen experiment to PS/PVME.92 From the cusps of the 'H wide-line spectrum of PS, they showed a certain amount of PS has intermediate or high mobilities. These studies and those in Section 4.5 show that miscibility found by the spin-diffusion studies does not necessarily guarantee homogeneity of local environments. A distribution of correlation times in a blend has not been appreciated in the studies in Sections 5.1 and 5.2. For example, the 13C linewidth studies on PS/PVME90.91do not show any asymmetric temperature dependence,

126

K. TAKECOSHI

which would be a consequence of equation ( 5 ) . Further, a 13C NMR lineshape under MAS and DD at higher temperatures above Tg can be satisfactorily simulated by a Lorentzian. Signals from fast- or slow-moving I3C spins appear as a sharp peak at the centre or a broad signal at the envelope of the signal. Therefore, 13C high-resolution NMR spectra are not sensitive to motional heterogeneity.

ACKNOWLEDGEMENT The preparation of this review was facilitated by the cooperation of Dr A. Asano who kindly helped me to survey references.

NOTE ADDED IN PROOF I notice that I have omitted several works. Maunu et al. studied miscibility of PAA/PVP, PANPEO, PEO/PMAA, and PMAA/PVP by the 'H relaxation times and 13C lineshapes. 11' Nzudie et al. examined PB/PMMA by the 'H TlP.lI8 Ageing and phase separation of PIP/PS have been investigated by 'H NMR imaging."' A 129Xe2D exchange NMR experiment has been applied to PS/PVME and PVC/PVME12' and also to PP/PP-PE. 12'

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One-bond 13C-13C Spin-Spin Coupling Constants K. KAMIENSKA-TRELA Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44, Warsaw 01-224, Poland

1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13.

Introduction Theoretical considerations Unsubstituted hydrocarbons Substituent effects on one-bond CC spin-spin couplings across single, double and triple bonds One-bond CC spin-spin coupling constants in derivatives of benzene One-bond CC coupling constants in heteroaromatic systems One-bond CC couplings in substituted aliphatic cyclic and heterocyclic systems 7.1. Three-membered ring compounds 7.2. Four-membered ring compounds 7.3. Five-membered ring compounds 7.4. Six-membered ring compounds 7.5. Large ring and polycondensed cyclic compounds The lone pair effect One-bond CC couplings in structural studies of complexes One-bond CC couplings in charged molecules and some related compounds One-bond CC couplings in biological studies Experimental methods Application of the INADEQUATE method in structural elucidations References

131 132 140 144 153 157 161 161 169 172 172 176 180 186 196 200 212 216 222

1. INTRODUCTION Lynden-Bell and Sheppard' and Graham and Holloway2 were the first to determine one-bond l3C-I3C couplings in ethane, ethylene and acetylene using 13C enriched samples, over 30 years ago. Developments in NMR instrumentation combined with the introduction of the Fourier transform (FT) pulse technique now allow one to measure 13C-13C couplings at natural abundance of 13C isotope. The pulse sequences, such as the one- and two-dimensional INADEQUATE (Incredible Natural Abundance DoublE ANNUAL REPORTS ON NMR SPECTROSCOPY VOLUME 30 ISBN 0-12-505330-4

Copyrighi 0I995 Academic Press Limited A / / rights of reproduction in any form reserved

132

K . KAMIENSKA-TRELA

QUAntum Transfer Experiment) technique and its further improvements and/or substitutes, simplify interpretation of the spectra considerably. This has made 'J(CC) couplings easily accessible, even in the case of large molecules, and revived interest in this unique parameter. It has occurred that in spite of initial assumptions, one-bond carbon-carbon couplings undergo strong variations upon substitution and complexation, and are in this way a sensitive measure of the electronic structure of carbon-carbon bonds. It is therefore not surprising that they have become a subject of lively interest, and a steady increase in the number of the papers devoted to 'J(CC) has been observed since the early 1980s. The present review covers the results which were published between 1987 and the end of 1993, with special attention paid to the factors which determine the 'J(CC) magnitude. The earlier results were covered in several exhaustive reviews written by M a ~ i e l M , ~a r ~ h a l lHansen ,~ and Wray,'-* and by Krivdin and Kalabin' and It is worth while to mention that the 13C-13C couplings across more than one bond were recently also reviewed by Krivdin and Della. l7 2. THEORETICAL CONSIDERATIONS A general theory of spin-spin coupling was developed in the early 1950s by Ramsey,ls*" who showed that indirect magnetic coupling between nuclear spins A and B results from three types of interactions:

J

= 'JoD

+ 'JsD +'JFC

(1)

those involving nuclear spin and electron orbital magnetic moment, 'JoD, dipole-dipole interaction between nuclear and electron spins, 'JsD, and the Fermi contact interaction, 'JFC, between the latter spins. Thus, electrons are involved in all interactions as a transmitting medium for the indirect coupling between nuclear spins. McConnell,20~21 Pople and c ~ - w o r k e r s , ~and ~ - ~Blizzard ~ and further developed Ramsey's theory and applied it to larger molecules. Though the theory was applied at an approximate level only, involving, among others, the so-called average excitation energy approximation, it afforded a successful interpretation of the FC, SD and OD contributions in terms of commonly used characteristics of the ground electronic states of the molecules under study. Thus, it has been shown that the contact contribution depends on the product S2(A).S2(B) of the electron densities at the coupled nuclei; the orbital and dipolar interactions are proportional to the product of the one-centre integrals, (rA-3)and (rB-3) for the coupled nuclei, where (rF3) is the expectation value of the inverse cube of the valence p electron radii. The first large sets of theoretical one-bond CC spin-spin coupling

ONE-BOND 13C-13C SPIN-SPIN COUPLING CONSTANTS

133

constants were published in the early 1970s by Maciel et a1.,27p30who calculated the SCF INDO values of one-bond CC couplings, 'J(CC), for a variety of organic molecules. In spite of the fact that the computations were performed for the Fermi contact term only, the agreement between the theory and experiment was in most cases satisfactory. The values of non-contact terms for CC spin-spin couplings have also been obtained by Blizzard and Santry26 from SCF perturbation theory calculations at the INDO level, and by Schulman and Newton.31 The older results have been collected and discussed by Kowalewski in his excellent review on calculations of nuclear spin-spin coupling constants.32 Furthermore, the mechanism of the spin-spin coupling has been a subject of continuing interest and in recent years there have been numerous papers on this t o p i ~ . ~ ~ Among - ~ l them, those devoted to the density functional calculations are of special interest. Recently, the first set of spin-spin coupling constants calculated by the use of this method has been published by Malkin et al.33 It includes one-bond carbon-carbon spin-spin coupling constants in acetylene, ethylene, propene, butadiene, benzene and pyridine. In the case of large molecules, especially those bearing the atoms of the second and the third row at the carbon-carbon bond involved, the INDO calculations still remain the method of choice. 34-50 The ab initio calculations are still rather time-consuming and require access to powerful computers. As a consequence only rather small molecules can be calculated by means of these methods. Nevertheless, the number of the papers devoted to the couplings calculated by these methods constantly i n ~ r e a s e s . ~ l -Recently, ~l the finite-field perturbation theory combined with the quadratic configuration interaction has been applied by C a r m i ~ h a e to l~~ calculate the Fermi contact contribution to the indirect CC spin-spin coupling constants across one bond in ethane, cyclopropane, cyclobutane and bicyclo[l.l.O.]butane. The computed 'J(CC) values matched the experimental ones very well. The ab initio data for cyclopropene have been recently published by Fronzoni and gal ass^^^ and by Barszczewicz et al.,60 and the ab initio value of 'J(CC) in vinyl lithium has been calculated by Ruud et a1.6' The most relevant data for the simplest molecules, i.e. ethane, ethene, ethyne and some cyclic unsubstituted compounds are collected in Table 1. The following general conclusions can be drawn from these data: (1) In all cases, without any exception, the Fermi contact term is the dominant factor. This also includes the coupling across central bond in bicyclobutane. The ab initio calculations performed by Galasso5' and by C a r m i ~ h a eshow l ~ ~ that even in the case of this unique bonding the coupling mechanism is controlled by the Fermi interaction, whose sign is, in this case, exceptionally negative. This leads to a negative sign of the 'J(CC) coupling constant, in agreement with the experimental

134

K. KAMIENSKA-TRELA

Table 1. Contributions from different mechanisms to the carbon+arbon couplings in ethane, ethene, ethyne and some alicyclics calculated by various semiempirical and ab initio methods; all values are in hertz.

Contact

Dipolar

35.6 41.5 41.5 43.24 30.24 34.1

0.7 1.6

CH3-CH3 SCPT INDO SCPT INDO SCF INDO FOPPA INDO/MCI EHMO Ab initio QCISD(T)

-

-1.45 0.18

Orbital -2.9 -2.9 -

0.59 0.04

Experimental HzC=CH2 SCPT INDO SCPT INDO SCF INDO FOPPA INDO/MCI EHMO Density functional results F32 F64 Ab initio results FPMC + CI" SOPPA".b SOPPA".' EOM Experimental

Ref.

33.4 40.2 41.5 42.38 30.46

26 50 27 43 42 58

34.6 70.6 82.1 82.2 84.22 74.90

81.9 98.6 78.8 86.99

HC-CH SCPT INDO 140.8 SCPT INDO 136.1 SCF INDO 163.6 FOPPA/INDO/MCI 164.49 EHMO 157.13 Density functional results F32 F64 A b initio results 173.81 FPMC + CI" SOPPA",' 194.2 SOPPA".' 186.7 SOPPA' 183.34 ECCDPPA 180.83 EOM 210.05 Experimental

Total

3.9 7.8 -

2.09 1.64

-

2.5 2.3 1.85

-18.6 - 18.6 -

-6.96 -3.97

-10.0 -9.3 -9.0 -6.48

55.9 71.3 82.2 79.35 79.57

26 50 27 43 42

70.8 61.2

33 33

71 .9 91.8 72.1 82.37

51 52 52 56

67.6 8.3 16.6

23.6 23.6

-

-

3.79 5.30

1.09 10.32

-

6.4 6.6 7.24 7.25 5.20

15.3 1.8 2.5 5.82 5.40 1.74

1

1

172.7 175.8 163.6 169.37 172.75

26 50 27 43 42

250.8 249.1

33 33

189.1 202.4 177.8 196.4 193.5 216.99

51 52 52 54 54 56

171.5 170.6

1 2

ONE-BOND I3C-l3C SPIN-SPIN COUPLING CONSTANTS

135

Table l-cont’d Contact Cyclopropane SCPT INDO A b initio QCISD(T)

23.6 13.9

Dipolar -3.6

Orbital

Total

Ref.

-0.6

19.4

50 58

12.4

62

Experimental Cyclobutane Ah initio QCISD(T)

27.4

58

Experimental (for methylcyclobutane) Bicyclobutane (edge) SCPT INDO A b initio QCISD(T)

25.2 24.3

-3.3

-0.5

29.1 28.9

63 64

21.4

31 58

Experimental Bicyclobutane (bridge) SCPT INDO A b initio EOM Ab initio QCISD(T)

65 -1.4 -7.0 -13.6

-1.3 -0.5 -

-2.9 -0.27

-5.6 -7.77 -

31 57 58

-17.5

66

-

Experimental (in 2,2,4 ,4-tetramethylbicyclobutane-dl 4) Cyclopropene (double bond) SCPT INDO 64.5 69.76 A b initio EOM Ab initio MC TDHF 72.8

9.2 -4.0 3.8

-6.5 1.8 -5.3

67.2 67.6 71.2

50 59 60

Cyclopropene (single bond) 25.0 SCPT INDO Ab initio MC TDHF 18.7

-1.0 -0.4

-2.7 -0.5

21.3 17.8

50 60

“Re-calculated from the reduced coupling constants K . bDZ basis. ‘DZP basis. Acronyms SCPT, Self-consistent perturbation theory; SCF, Self-consistent field theory; LMO, Localized molecular orbitals; EHMO. Extended Hiickel molecular orbital theory; FOPPA/INDO/MCI, First-order polarization propagator approach (multicentre integrals); FPMC CI, Finite perturbation-multiconfiguration SCF method plus configuration interaction; SOPPA, Secondorder polarization propagator approximation; ECCDPPA, Extended coupled-cluster doubles polarization propagator; EOM, Equations of motion; QCISD(T), Quadratic configuration interaction with the space restricted to single, double (triple) substitutions; MC TDHF, Multiconfiguration time-dependent Hartree-Fock approach.

+

136

K. KAMIENSKA-TRELA

Table 2. FFT INDO-calculated values of one-bond CC coupling constants in the derivatives of acetylene (in hertz).

Compound HC=CLi HC=CBeH HCECBH~ HCzCCH3 HGCNH2 HC=COH HC-CF HC=CNa HC=CMgH HC-CAlH2 HC=CSiH3 HCECPH~ HC-CSCH3 HC-CC1 LiCICLi HBe-C=CBeH H~B-CECBH~ H~CCECCH~ HZNGCNH2 HOC=COH FC=CF H3SiC=CLi H3SiCECBeH H3SiC=CBHz H3SiC=CCH3 H3SiC=CNH2 H3SiC=COH H3SiC=CF H3SiC=CSiH3

Contact

Dipolar

Orbital

Total

Ref.

51.6 86.0 125.5 162.0 179.7 190.6 200.0 56.5 69.9 93.6 125.2 145.5 164.6 180.9 31.4 47.1 102.5 161.0 202.0 235.3 275.5 41.6 65.8 97.3 123.2 132.6 135.8 135.1 96.2

4.8 4.4 4.8 5.7 5.7 6.0 6.0 5.1 4.9 5.1 5.7 5.0 6.1 6.3 2.1 0.7 5.5 7.5 9.3 10.5 11.1 4.6 4.4 4.5 5.5 5.5 5.8 5.9 5.5

5.0 4.4 6.1 7.9 8.4 9.3 9.8 5.8 5.5 6.2 7.7 7.6 9.7 10.6 3.7 3.5 4.8 5.6 6.0 6.4 6.4 4.7 3.8 5.1 7.2 7.8 8.8 9.3 7.4

61.4 94.8 136.4 175.6 193.8 206.0 215.8 67.4 80.3 104.9 138.6 158.1 180.5 197.8 37.2 51.3 112.8 174.1 217.3 252.2 293.0 50.9 74.0 106.9 135.9 145.9 150.4 150.3 109.1

45 45 45 45 45 45 45 49 49 49 44 49 49 49 45 45 45 45 45 45 45 49 49 49 49 49 49 49 44

result reported by Finkelmeier and Luttke for perdeutero-2,2,4,4tetramethylbicyclobutane.66 In all remaining cases known so far the couplings across one CC bond are positive. (2) The orbital and dipolar terms are negligible for single CC bonds. For the double CC bond the non-contact terms are of mutually opposite signs and partly cancel each other. Only in the case of the triple CC bond, the orbital as well as the dipolar terms are large and positive. In Tables 2-4 the INDO 'J(CC) values calculated for substituted acetylenes, ethylenes, allenes and diacetylenes, where substituents were systematically vaned, are displayed, and some of these results are additionally shown in Figs 1 and 2. Though the agreement between the calculated and experimental coupling constants (the latter values are

137

ONE-BOND 13C-'3CSPIN-SPIN COUPLING CONSTANTS

Table 3. The I N D O FPT total 'J(CC) values in variously substituted ethenes, allenes and diacetylenes; for H,C=CHLi the ab initio 'J(CC) value taken from ref. 61 is included.

XCH=CHz XC'H=C2=C3H2 Substituent X

'J( C C ) ~ ~ 'J( c1c 2 y 7

14.6 Li 51.3" Li" 35.4 BeH 68.5 BH2 82.1 CH3 93.3 NH, 96.8' OH 95.0" OH 99.6 F Range of O D contribution -5.4to -6.5 0.9 to 1.0 Range of S D contribution ~~

XC'd?-C3zC4X l ~~(

1

~

I J (2 ~

) 2 ~ ~ ~3 ) ~

27.3

43.2

104.0

55.1 75.7 107.5 120.1 128.8' 125.4' 130.8 --3.7 to -4.4 1.6 to 2.9

93.5 137.5 178.9 202.1 216.5

118.3 135.3 154.0 165.0 170.7

-

~

-

-

-

-

233.3 176.1 3.0 to 10.1 -0.1 to 0.3 4.1 to 6.0 -0.3 to 1.0

~

"Ab inrtio value ('J(FC) 57 83. 'J(0D) 9 53, 'I(SD) 3 07) 's-cry arranged OH group 's-trans arranged OH group

Table 4. Calculated total INDO FPT and FOPPA INDO/MCI one-bond CC couplings in fluoroethenes; all values in hertz.

Compound

H2C=CF2 FCH=CHF cis FCH=CHF trans FCH=CF2 F,C=CF, Range of OD contribution Range of S D contribution

FOPPA INDO/MC143

INDO FPP7

116.4 109.8 121.3

119.2 113.5 129.4 146.6 196.4 -3.6 to -4.8 2.5 to 3.3

-

-5.6 to -6.2 2.4 to 2.8

collected and discussed in Section 4) is not always satisfactory, some general trends emerge from the data collected. Thus, in all cases, without exception, a strong influence of electronegativity of substituent is observed. The largest calculated values have been found for fluorosubstituted compounds, the smallest for derivatives of lithium and sodium. The dramatic changes observed can be interpreted mainly in terms of the changes occurring in the Fermi contact term. The contribution of orbital-dipole and spin-dipole interactions, though not negligible, is considerably smaller than that introduced by the FC interaction. This is in agreement with the semiempirical data published earlier by Maciel et for some substituted

138

K. KAMIENSKA-TRELA

___c_c_c_.

(OD+SD)

0 L i B e B C

N

O

0 F

(atomic number) Fig. 1. Plot of the total, the Fermi contact (FC) and the sum of orbital and dipolar terms vs. atomic numbers of the first atom of the substituent for 'J(CC) coupling constants in disubstituted acetylenes.

ethenes and acetylenes and with the ab initio results published by Fronzoni and Galassos9 for monoheteroanalogues of cyclopropane and cyclopropene. The theoretical calculations of one-bond CC coupling constants performed at both semiempirical and ab initio levels predict for them a dependence on both conformation and configuration of the compounds. Thus, variations by as much as 5-6Hz were noted by Barfield and co-workers68 in the INDO values of 'J(CC) couplings as the dihedral angle 4 was varied in butane, butanol and butanoic acid. A very strong 'J(CC) vs. the valence angle dependence has also been observed by Krivdin et ~ 1 . ~who ' performed the calculations for the model CH2CH2CH2fragment using the SCPT INDO method. The ab initio calculations performed by Carmichael et al.69 at the


139

H 3SiCCX

150 -

100

-

50 -

0

L i B e B C

N

O

F

(atomic number) Fig. 2. Plot of the total, the Fermi contact (FC) and the sum of orbital and dipolar terms vs. atomic numbers of the first atom of the substituent for 'J(CC) coupling constants in silyl-substituted acetylenes.

QCISD(T) level for ethylene glycol revealed that 'J(CC) coupling in this compound depends on both the C-C and C - 0 torsions. The coupling is largest when the hydroxyl substituents are trans and smaller for gauche geometries; the coupling reaches a maximum when the hydroxyl proton is anti to a carbon and a minimum in gauche configurations. Thus, in addition to the relative C-C torsion one should also take into account the conformational behaviour of the C - 0 bonds. 'J(CC) values found for trans 1,2-difluoroethene were substantially greater than those calculated for the cis-difluoro i s ~ m e r . ~ ~ ? ~ '

140

K. KAMIENSKA-TRELA

3. UNSUBSTITUTED HYDROCARBONS

A large number of the 'J(CC)couplings have been collected for a variety of unsubstituted hydrocarbons and thoroughly discussed in two recent reviews.9310Therefore, in this section only some of the most important data are included in order to allow the reader to become quickly familiar with the general trends prevailing in this group of the compounds. Two factors have to be taken into consideration in order to estimate the unknown one-bond CC coupling value. These are (i) hybridization of the bonding orbitals involved and (ii) the ring size of the compound. With the increasing s characters of carbon atoms involved the magnitude of the corresponding coupling increases. The couplings across endo-cyclic carbon-carbon bonds increase with increasing ring size, while the reverse trend is observed for the couplings across em-cyclic bonds which decrease upon passing from smaller to larger rings: 'J(Csp3Csp3)< 'J(Csp3Csp2)< 'J(Csp2Csp2) (single bond) = 'J(Csp2Csp2)(aromatic bond) = 'J(Csp3Csp)< 'J(Csp2Csp) (single bond) < 'J(Csp2Csp2) (double bond) < 'J(Csp2Csp)(allenic bond) < 'J(CspCsp) (single bond) < J(CspCsp) (triple bond) 'J(CC) endo in C3H6< 'J(cc) endo in C4Hs< 'J(CC) endo in CSHlo= 'J(CC) endo in C6H12 and 'J(CC) ex0 in C3H5CH3>'J(CC) ex0 in C4H7CH3='J(CC) ex0 in C5H9CH3= 'J(cC) ex0 in C6HllCH3

Table 5 and Fig. 3 illustrate these trends. The results obtained recently by Eckert-Maksic et d7' for allenes substituted with a cyclopropyl ring provide a particularly elegant illustration of the above trends. The couplings of 151.8Hz, 142.2Hz and 142.0Hz observed in (1) bis(cyclopropy1idene)methane (2) bis(tetramethylcyclopropy1idene)methane

(3) 1-(dimethyletheny1idene)cyclopropane

across cyclopropyl substituted allenic bonds are greater by ca. 44-54 Hz than the coupling in unsubstituted allene (98.7 Hz):

B=-.==q 4

3

2 '

1

2 1

3

141


151.8Hz

'J(C2C3) 'J( C3C4)

(3)

(2) 23.0 Hz 142.2 Hz 142.2 Hz

(1) 24.6 Hz

'J( ClC2)

151.8 Hz

45.0 Hz 109.0 Hz 142.0 Hz

Thus in the absence of strongly influencing factors such as complexation, electron-donating o r electron-attracting substituents, the hybridization of

Table 5. Relationship between 'J(CC) couplings and hybridization.

Hybridization of carbons involved

Compounds

3 3 SP3> S P , SP , S P

CH3CH3 CH3CH=CH2 CH2=CH-CH=CH2 C6H6

sp:, sp2 (single bond) sp ,sp2 (aromatic bond)

H2C=CH2 H~C-CECH H,C=CH-C-CH H&=C=CH2 HCEC-CECH HC=CH

I(ClC2)

12.4

SP2, SP2

sp3, sp (single bond) sp2,sp (single bond) S P 2 , sp sp, sp (single bond) S P , SP

32.7

!J(C 1c 1,)

refs

62

62

'J(CC)

Ref.

34.6 41.9 53.7 55.95 55.88 67.6 67.4 83.9 98.7 154.8 171.5

1 29 70 71 72 1 73 73 74 75 1

13.3

29.1

43.4

36.1

36.2

35.7

40,64

64

64

64,76

33.3

lJ(CIC1')

95 2

73 1

73 8

72 3

lJ(C1C 2)

23 2

33 6

39 6

39 7

28 5

33 3

31 9

78

78

78

'J(C2C3) refs

77

Fig. 3. Influence of ring size on one-bond CC coupling constants.

142

K. KAMIENSKA-TRELA

Table 6. Collection of the coefficients in the modified versions of equation (2). Coefficients U

576 598 621 658 637

'J(CC)values applied

Ref.

Experiment Experiment

80 77 81 62 15

b

3.4 3.5 10.2 7.9 11.0

INDO

Experiment Experiment

bonding carbon orbitals is the factor which dominates the magnitude of carbon-carbon spin-spin coupling. The corresponding equation of the form where sA and sB are the s characters of intervening carbon atoms was derived by Frei and Bernstein in 1963;80since then several slightly differing values of numerical coefficients entering this equation have been proposed. The authors used either theoretical or experimental sets of 'J(CC) couplings. The corresponding a and b values are collected in Table 6. Some of the authors have modified the equation (2) including overlap integrals (,SAB) calculated using the maximum overlap

The Frei-Bernstein relationship has been recently applied by Jarret and C u ~ u r n a n o ~in~ order to estimate the orbital hybridization in [l.l.l]propellane (4). The one-bond C1C2 coupling of 9.9Hz was found in indices for the C-CH2 bond, in good this compound leading to sp8.6-~p4.8 agreement with the earlier theoretical treatment. Finally, it should be added that determination of carbon-carbon coupling constants for such basic molecules like benzene, cyclohexane, cyclopropane, etc. is by no means a trivial task. Different approaches were applied in order to overcome difficulties arising from the symmetry of the spin systems.9 have applied the method of isotopic Recently, Roznyatovsky et perturbation of symmetrical spin systems by H/D substitution in order to measure one-, two- and three-bond CC couplings for benzene and cyclohex-

143

ONE-BOND 13C-13C SPIN-SPIN COUPLING CONSTANTS

Table 7. One-bond spin-spin couplings across one-, two- and three-bonds in benzene and cyclohexane; all values are in hertz. Compounds

'J(CC)

'J( CC)

3J(CC)

Ref.

Benzene Benzene" Benzeneb Cyclohexane Cyclohexane

55.88 55.3 0.5 55.95 k 0.04 33.10 32.7

2.46

10.04 10.08 0.10 10.01 k 0.03 2.05

72 86 71 72 62

*

-

2.12 -

*

-

"Measured from the nematic phase. bThe method of measurements not reported.

Tables. One-bond carbon<arbon pentadienes.88 All values in hertz.

No. 6

7 8 9

couplings in polymethylsubstituted

cyclo-

R

'J(ClC2)

'J(ClC.5)

'J(ClC1')

'J(C2C2')

'J(C5C5')

H D CH3 NO2

71.0 71.4 71.4

40.7 40.7 40.7 43.3

47.8 47.5 47.5 48.7

45.4 45.3 45.3 45.3

33.6 33.1 33.1 37.4

-

ane. The measurements using double quantum filtration and subsequent analysis of the spectra were performed for monodeuterated derivatives of these two compounds giving the very precise 'J(CC) values. The results reported by Roznyatovsky et aL7' (Table 7 ) for benzene are in agreement with those reported earlier by Diehl et ~ 1and. by~Wray ~ et ~ 1 .the ; ~'J(CC) ~ coupling for cyclohexane is in accord with that published by Luttke and his co-workers.62 An interesting result has been reported by Gay et U I .who ~ ~ have recorded the INADEQUATE spectrum of adamantane (5) in the solid state. The one-bond CC coupling of 31.4 k 0.5 Hz observed in this spectrum was in good agreement with the value of 31.5 Hz obtained by the same authors in benzene solution.

144

K. KAMIENSKA-TRELA

H

H Finally, 'J(CC) couplings have been published by Borodkin et aZ.** for some polymethyl substituted cyclopentadienes (Table 8). 4. SUBSTITUENT EFFECTS ON ONE-BOND CC SPIN-SPIN COUPLINGS ACROSS SINGLE, DOUBLE AND TRIPLE BONDS

The electronegativity of substituents is a second important factor which, as theoretical data presented in Section 2 showed, determines the magnitude of one-bond CC couplings. In particular, the experimental 'J(CC) data obtained for a large number of variously substituted acetylenes fully corroborated the theoretical predictions made for this group of compounds. The data collected in Tables P-11 show that indeed the couplings across the triple bond are extremely sensitive towards substitution. They extend over more than 170Hz varying from 56.8 Hz in (C2H5)3SiC=CLiS9to 230.4 Hz in m-CH30C6H50C=CCH3.90 A lack of the corresponding data for the couplings across single and double bonds led some authors to somewhat hasty conclusions that the couplings across triple CC bonds are an exception, whereas all the remaining ones are much less affected by substituent effects. However, the most recent data obtained for halogenosubstituted ethanes by Gornb1e1-l"~ and for variously substituted ethenes by Kamienska-Trela et al. lo5 and by Wrackmeyer et al. ,lo6 revealed that these coupling constants also undergo a dramatic change upon substitution. The 'J(CC) coupling across double CC bond of 30.9Hz only has been found in (Me3Sn),C=C(Et)BEt2 by Wrackmeyer et al. ,lo6 and 'J(CC) of 172 Hz for trifluorochloroethene by Kamienska-Trela et al. lo5 leading to the total range covered by these couplings larger than 140Hz. A very small 'J(CC) coupling constant of ca. 36Hz only, has been reported recently for vinyllithium by Kamienska-Trela et af.lo7 and by Bauer and Griesinger. The coupling of 85Hz in F3CCC13 is the largest 'J(CC) value across a single Csp3Csp3bond reported so far.104This, combined with the value of 22.9Hz in (CH3)2CHLi,109 gives the total range of ca. 60Hz. Similar changes are observed for the couplings across Csp3Csp2 (0)bonds as shown

ONE-BOND l3C-I3C SPIN-SPIN COUPLING CONSTANTS

145

Table 9. One-bond CC coupling constants, 'J(CC), across triple bonds in XC=CY acetylenes; all values are in hertz.

Y Substituent X ~~

=

J

Ph

Y Ref.

OMe NEt2 Br Ph EtCO,CH=CH" SMe I Et SeMe PPh2 PBu~ TeMe' SiMe3 GeEt, SnEt, PbEt, Li Y X OEt NEt, %Me3 SnMe3 PbMe3 PbPh3 m-CH30C6Hs

Y = Me3Si

J

Ref.

J

Ref.

-

-

-

-

216.0 204.3 202.5 185.0

45, 89 48, 89 45, 89 92

204.8 -

89

-

166.7 155.3

89 45, 89

190.6

45, 89

143.2

45, 89

-

-

-

-

184.2 179.7 177.6 173.2 157.0 154.1 154.4 136.9

45, 91 45 89 89, 91 48 48, 89 89, 91 89

175 175.0 169.5 170.7

93 89, 91 45, 89 45, 89

-

=

-

-

-

-

134.2 126.6 133.3'

89 45, 89 45

115.26

48, 89

-

-

-

-

-

-

130.6 131.5 120.5 111.9

45 94 94 16

101.4'

89, 95

-

CH3

-

94.0

-

45 -

-

56.8

45, 89

J

Ref.

216.5 175.9 175.0 131.8 132.5 125.7 119.8 113.0

94 80 94 45 89 97, 98 95 96, 97

Y = H

J

Ref.

224.3 204.0 136.7 127.6 120.0 122.6 230.4

96 96 45 96, 97 96, 97 16 90

Y = SnBu3

SnBu3

C(CH3)3

~

c1

X

=

J

Ref.

81.0

95

"The trans compound. bMeasured for Et,Si derivative. 'In ref. 89, erroneously CH,TeC=CH was given.

X OEt Ph Me SiEt3 GeEt, HgMe SnBu3 PbEt,

146

K. KAMIENSKA-TRELA

Table 10. One-bond CC coupling constants, 'J(CC), in derivatives of diacetylene XIC-C-C=CX2; all values are in hertz.

xi

x2

'J(C=C)

Ref.

188.3 146.4 146.8 134.5 124.5 145.7b

75 100 101 101 101

99

"Couplings across central CC bond are given in Table 15 'Coupling in (CH,),SiC=C fragment.

Table 11. Influence of values are in hertz.

p substituents on 'J(CC) coupling constants in acetylenes; all

Compounds

'J(CC) Ref.

HOCHZCECH (CH,)*C(OH)C=CH (CH3)2C(OH)C=CCl (CH3)*C(OH)C=CBr C~HSOCH~CECH CHzCICeCH CH2BrC=CH CSF1 ICECH

169.3 166.3 202.2 188.4 173.1 178.6 179.3 188.5

90 90 90 90

90 90 90 96

Compounds

'J(CC) Ref. 146.0 150.6 136.0 133.3 131.3 130.4 174.0 170.7 177.0

90

90 102 102 102 102 102 102 103

-

by the data obtained by G ~ m b l e r "for ~ trifluoroacetic acid and its chloride (see Table 16). The experimental data available in the literature for substituted allenes are rather scarce and originate from two papers by Krivdin et aZ."O and Afonin et d.'" who measured the couplings for two series of alkoxysubstituted allenes. Also in this group of compounds with the formal Csp'Csp hybridization the expected sensitivity towards substitution is revealed by the one-bond CC couplings (Table 12). The following general conclusions and remarks can be made upon a careful inspection of the most recent data collected in Tables 9-17, as well as those presented in earlier reviews. Thus, within a given type of CC bond the electronegativity of asubstituent is the main factor controlling the magnitude of 'J(CC) coupling. The 'J(CC) values increase with increasing electronegativity of substituent, and diminish when the electronegativity of substituent decreases.


N

100.0

N

90.0

I

0 I

-0 F

147

1

-J

-0

c

0

n I

a,

5

30.0

1

I

I

I

I

I

I

I

0.5

1 .o

1.5

2.0

2.5

3.0

3.5

4.0

Pauling's electronegativity of substituent Fig. 4. The 'J(CC) vs. Ex relationship in monosubstituted ethylenes. The Pauling electronegativities Ex of the first atom of the substituent are taken from R. McWeeny, Coulson's Valence, p. 163, Oxford University Press, London, 1979.

An equation which has a multiplicative form has been derived for the couplings across the triple bond:89

' J ( X C = C Y ) = 23.23 (f0.51)ExEy+ 15.45 (f3.32)

( n = 27; r

=

(4)

0.99; s.d. = 4.2Hz)

where Ex and Ey denote Pauling's electronegativities of substituents attached at the triple bond. The substituents involved in the correlation represented a wide range of electronic interactions (C1, Br, I, Li, O R , SR, NR2, PR2, C(CH3)3, SiR3 and GeR3) and therefore one can expect that the validity of equation (4) should be quite general.89 Attempts to apply an additivity rule to these couplings have also been made' but they led to rather dubious results, since large deviations from this scheme have been found when strongly influencing substituents such as SiAlk3, SnAlk3, C1 and Br have been attached to the triple bond simultaneously. The 'J(CC) vs. Ex dependence in monosubstituted ethenes is obviously non-linear (see Fig. 4) and in the case of di-, tri- and tetrasubstituted ethenes also steric effects, i.e. configuration of the compound and conformation of substituents have to be taken into a c c o ~ n t . ~ ~ ' ~

148

K. KAMIENSKA-TRELA

Table 12. One-bond CC couplings (in hertz) in alkyl-, ROC'H=C2=C3H2(1&18), and vinylalkoxyallenes, H2Cp=CaH-O-C'R=C2=C3H2 (19-26).

Compound no. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

R CH3 C2H5 n-C3H7 i-C3H7 i-C4Hy CH2CH=CH2 t-C4Hy HC=CH2 C6H5

CH3 C2Hs n-C3H7 n-C4H9 i-C3H7 i-C4Hy 2;ec-C4H9 t-C,H,

' J ( ClC2)

'J( C2C3)

113.8 114.1 113.7 114.0 113.7 114.8 118.2 117.8 117.7 118.6 118.5 118.0 118.3 117.8 118.2 117.8 118.0

101.7 101.8 101.4 101.9 101.4 101.1 102.2 101.9 102.1 102.1 102.2 102.0 102.1 102.0 102.2 102.1 102.1

' I (CcuCp)

Ref.

-

110 110 110 110 110 110 110 110 110 111 111 111 111 111 111 111 111

-

-

-

81.2 -

81.5 82.0 82.4 82.1 81.7 82.2 82.1 82.2

Table 13. One-bond CC coupling constants, 'J(CC), in some selected monosubstituted ethenes H2C=CHX; all values are in hertz. ~

Substituent Li

'J(CC)

Ref.

35.8"

107 107 108 108 112 113 114 113 112 114 114

35.9h

36.3' 35.0d 54.7 58.8 58.2 58.2 60.6 60.3 61.3

Substituent

'J(CC)

Ref.

70.0 72.3 72.4 72.9 76.3 77.6 78.6 78.4 82.2

113 112 112 112 112 115 112 113 112

"Saturated solution of tetramer in EtZO (measured at ambient temperature). '3 M solution of tetramer in EtzO (measured at ambient temperature). '2.3 M solution of tetramer in THF-$ (measured at +30"C). d3.4M solution of dimer in C,D6 with 1.5 equiv. of TMEDA (measured at 3OOC).

ONE-BOND 13C-13CSPIN-SPIN COUPLING CONSTANTS

149

Table 14. 'J(CC) in trans- and cis-substituted ethenes (in hertz).

Substituents FHC=CHBr cis FHC=CHBr trans CIHC=CHCl cis ClHC=CHCI trans BrHC=CHOC2HStrans BrCH=CHOC2Hs cis BrPhC=CHBr cis BrPhC=CHBr trans BrHC=CHBr cis BrHC=CHBr trans IHC=CHI cis IHC=CHI trans C1((CH&Si)C= CHCl cis CI((CH3)3Si)C=CHCltram Br((CH&Si)C= CHBr cis Br((CH3)3Si)C=CHBrtrans (CH3)3SiCI=CHIcis (CH3)3SiCI=CHI tram CH3CH=CHBr cis CH3CH=CHBr trans (CH3)3SiCH=CHClcis (CH3)3SiCH=CHCltrans (CH&SiCH=CHBr cis (CH3)$iCH=CHBr trans CI(CH3)2SiCH=CHCIcis Cl(CH3)2SiHC=CHCItram C13SiCH=CHCI cis Cl3SiCH=CHC1trans (C2HsO)(CH3)2SiCH=CHClcis (C2Hs0)(CH3)2SiCH=CHCltrans (C2H50)2(CH3)SiCH=CHCI cis (C2Hs0)2(CH3)SiCH=CHCI trans (C2H50)3SiCH=CHClcis (CH30)3SiCH=CHClcis (C2Hs0)3SiCH=CHCl trans (CH30)3SiCH=CHCltram C6Hl CH=CHSi(CH& cis C6H1'CH= CHSi(CH3)3trum

'J(CC)

Ref.

87.9 96.0 84.5 91.9 91.5 k 0.5 88.3 k 0.5 87.0 89.5 82.2 86.4 78.7 78.3 67.5 77.5 64.9 72.4 62.5 66.3 78.0 77.0 70.9 65.5 k 0.5 69.9 64.5 0.5 71.1 67.0 72.3 69.7 70.9 65.1 70.3 64.9 70.8 70.7 65.4 65.4 61.4 60.6

105 105 116 116 105 105 105 105 117 117 118 118 117 117 117 117 117 117 105 105 114 114 114 114 114 114 114 114 114 114 114 114 114 114 114 114 119 119

*

150

K. KAMIENSKA-TRELA

Table 15. One-bond CC coupling constants in variously substituted ethylenes; all values are in hertz.

Compounds F2C=CFCI F2C=CC12 PhCHzCFZ CI3SiCCI=CCl2 CI2(CH3)SiCCI=CCl2 (C2H50)3SiCC1=CC12 (CH3)3SiCCI=CC12 CI3SiCH=CCl2 C12(CH3)SiCH=CC12 Cl(CH3)2SiCCI=CC12 (C2H50)3SiCH=CC12 (C2H50)2(CH3)SiCH=CC12 CH3CBr=CH2 (CH3)3CC-CCH=CHCN trans ( CH&CC-CCH=CHC02Et trans PhCH=CHC02Me (C2H5)3SiCH=CHCOOC2H5 trans (CH3)$KBr=CH2 (CH30)3SiCH=CHCItrans (CH3)2HSi(CH3)C=C(B(C2H5)2)C2H5 cis (CH~).~S~(C~HI~)C=C(B(C~H~)~)C~H~ cis (CH3)3SnCH=C(B(C2H5)2)C2H5 cis (CH3)3SnCH=C(Sn(CH3)3)C2Hs cis C5Hll((CH3)3Si)C=CHSi(CH3)3trans

(CH3)3Si((CH3)3Sn)C=C(B(C2H5)2)C2H.iU

((CH3)3Si)2C=C(B(C2H5)2)C2H5 ( (CH3)3Si)2C=CHSi(CH3)3 ((CH3)~Sn)2C=C(B(C2H5)2)c2HS

'J(CC)

Ref.

172 k 1 154 2r 0.5 115.3 90.0 88.1 84.5 82.4 82.5 80.7 79.5 79.6 78.2 79.0 k 0.5 77 74 72 65.3 62.8 2r 0.5 65.4 56.1 57.2 54.6 54.6 53.2 k 0.5 51.5 50.8 45.3 k 0.5 30.9

105 105 47 114 114 114 114 114 114 114 114 114 105 93 93 93 114 105 114 120 120 106 106 105 106 106 105 106

"Silyl and ethyl substituents trans.

The data obtained for cis and trans 1,2-dihalogenoethene~~~"~~~ and for cis and trans l-trimethylsilyl-l,2-dihalogenoethene~~~~ showed that 'J(CC) couplings in trans compounds are larger than those in cis ones, but the difference diminishes with decreasing electronegativity of halogen, and becomes negligible for iodine substituent. In l-trimethylsilyl-2halogenosubstituted and in 1-trimethylsilyl-2-alkylsubstituted compounds the opposite trend is ~ b s e r v e d ; "the ~ couplings in cis compounds are greater than those in trans ones. The data available in the literature indicate that cis dihalogenosubstituted ethenes are more stable than trans ones, but the difference, AE, decreases in order 1,2-difluoro- (3.9 kJ/mole), 1,Zdichloro(2.7 kJimole), 1,2-dibromo- (1.3 kJ/mole) ethene and is close to zero for

ONE-BOND 13C-'3CSPIN-SPIN COUPLING CONSTANTS

151

1,2-diiodoethene~.'~~~~~~ No theoretical justification could be offered for the observed relationship between A'J(CC) and AE. Since, however, 'J(CC) coupling constants depend on the contributions of the s electrons to the corresponding bonding orbitals, the parallel decrease in A'J(CC)s and AEs can be tentatively associated with the s-electron densities at the bonding orbitals of the cis and trans isomers. It can be also concluded that, by analogy, the larger 'J(CC) value indicates the more stable isomer when two analogous compounds are compared. An influence of strongly electronegative substituents in the P position to the CC bond is considerably smaller than that of a-substituent. Moreover, as the studies performed by Krivdin et ~ 1 . ~ for ' a series of substituted acetylenes showed, no simple correlation exists between the nature of the P-substituent and its influence on the 'J(CC) coupling concerned. An average increase of 3 to 5 Hz in 'J(CC) is observed upon the introduction of one chlorine or bromine atom, but a decrease of similar magnitude is found when hydroxy or phenoxy groups are introduced. A small but systematic decrease in 'J(CC) has been observed upon the introduction of subsequent P-methyl groups in the alkyl substituent in triethylsilyl acetyleneslo2 (Et3SiC= CAlk), but no analogous regularity is revealed by t-butylacetylenes (t-BuCECAlk) (see Table 11). Particular attention is invited by the results obtained by Lambert and Singer,13' who measured the 'J(CC) couplings for a series of the benzyl derivatives of the type X-C6H4CH2M(CH3)3 (M = Sn, Ge, Si, C and X = H, Me, OCH3, CN) (Table 18). The 'J(CipsoCH2) couplings in these compounds decrease in order Sn > Ge > Si > C (ca. 43 Hz, ca. 43 Hz, ca. 41 Hz, 36 Hz, respectively). This result has been invoked by the authors as evidence for the presence of hyperconjugation in the neutral ground state in agreement with the double-bond, no-bond valence structure: X--CgHd=CH2 +M(CH3)3 However, among the results reported, those obtained for XC6H4CH2C(CH3)3draw attention. All previously reported 'J(CC) couplings for analogous compounds are in the range 43 to 45Hz: PhCH3 (44.2 H z ) , ~ PhC2H5 (45.5 H z ) , ~ PhCH(CH3)2 (43.3 H z ) , ~ PhC(CH,), (43.3 Hz) .5 This includes 'J(CipsoCHz) coupling in PhCH2CH(CH3)2 (43.5 Hz) (measured in our laboratory). Therefore, the values of 36 Hz found by Lambert and Singer for the series XC6H4CH2C(CH3)3(X = H, OMe, Me, NO2) seem to be rather improbable and obviously require re-investigation. The decrease of these couplings by comparison with all others is even less understandable in the light of the authors' statement'37 that the CHZC(CH3), group does not hyperconjugate while the other three, i.e. CH2Si(CH3)3, CH2Ge(CH3)3 and CH2Sn(CH3)3, where the rather typical 'J(CC) values are observed, do. See also "Note added in proof" on p. 221.

Table 16. One-bond CC coupling constants across single bonds; all values are in hertz.

Compound

'J(CC) Ref. Compound

Csp3Csp2bonds CH3CH=CH2 CH3CH=CHCHO CH3COCH2CH2CH=CH2 HOCH2CH=CH2 CH3CCl=CHp CH3CCl=CC12

'J(CC) Ref.

22.9 28.4 31.7 32.8 33.0 33.1 33.8 37.6 38.2

109 109 6 124 97 97 97 69 123

CH3CHOHCH20H HOCH2CH(OH)2 (CH3)3CF CC13CH3 CF3CD3 CF3CH3 CF3(CH2)30H CF3CBr3 CF3CC13

41.3 48.7 40.3 42.7 60.5 60.5 60.3 73.0 85.0

69 69 109 104 104 104 104 104 104

41.9 41.2 41.2 45.4 48.5 50.3

5 125 125 41 29 115

ClCH2CCl=CC12 C12CHCCl=CC12 C13CCCl=CC12 CF&H=CHZ CF,CCl=CCICF3

55.2 60.8 68.6 75.7 74.4

115 115 115 104 104

Csp3C(0) bonds CH3C(0)CH2Ra CH3C(0)CH2Ra CH3C(O)CH3 CH3C(0)ONa CH$(O)Cl CHSC(0)OH CH3(CH2)13C(O)ONa CF3C(0)CF3 CF&(O)OH CF3C(O)CI

38.6'." 125 40.4'~~ 125 40.44 5 +54' 126 56.1 5 56.7 5 +58 126 82.4 104 103.4 104 105.6 104

CH3COCH(CH3)C02GH5 56.6 CH3COC(CH3)2C02CH3 56.8 C H ~ C H O H C H ~ C O ~ C ~ H57.0 S C6HsCH2C02CH3 57.8 BrCH2CH2C02CH3 58.3 C H ~ C O C H ~ C O ~ C ~ H S 59.0 CH3C02CH3 59.8 59.7 CH3COC2HS 60.9 CH3CH(OH)C02C2HS NCCH2C02C2H.j 61.4 62.2 HSCH2C02CH3 CH3CHBrC02C2HS 64.4 ClCH2C02CzHs 64.7 BrCH2C02C2H5 65.0

127 127 127 127 127 127 127 127 127 127 127 127 127 127

Csp3Crp bonds CF3-C-CCF3 BrCH2-C=CH ClCH,-C=CH HOCH,-C=CH CH~-C=COC~HS CH3-C=CN(C2H5)2 CH3- C=CSi(CH3)3 CH3-C=CSi(C2Hs)3 CH3-CzCSn(CH3)3 CH3-C-CPb(CH3)3

133.9 78.0 77.5 71.5 74.8 70.0 63.5 63.2 62.2 59.0

(CH3)3C-C=CCl 68.5 (CH3)3C-C=CCZHS 68.3 ( C H ~ ) ~ C C E C - C H ~ C H ~ 68.0 (CH~)~C-CGCSCH~ 67.1 (CH&C-C=CBr 67.0 (CH3),CC=CCH=CHCO,Et 67.0 (CH3)3C-C=Cl 65.1 (CH3)3C-C-CSi( CH& 62.0 61.8 (CH3)3C-C-CSi(C2H5)3 (CH3)3C-C-CGe(CH3)3 62.5 (CH&C- C-CSn( C2H5)3 61.4 (CH3)zCH- C=CSi(C ~ H S ) ~62.1 CH3CH2-CZCSi(C2H& 62.3

129 45 45 91 45 93 45 45 45 129 129 45 45

104 128 128 128 96 96 45 45 96 96


153

Table 16-contd. Compound

Csp2Cspzbonds (CH3)2NC5H=C4H-C3H=C2H-C'Hd

_

_

'J(CC) Ref.

60.3

130

58.5 73.4 73.7 74.7 76

130 127 127 127 93

~

Csp2Csp bonds (CH3)3CC=C-CH=CHCO&HS (CH~)~CCEC-CH=CHCN

CspCsp bonds (CzHS),SiC=C-C=CSi(C2Hs)3 (CZH,)~G~C=C-C=CG~(C~H~)~

92 93

93 93

127.7 119.3 126.2 123.0

131 131 131 131

137.2 137.7

75 100

"R = CH2CH=CH2. bFor the C(0)CH2 fragment. "J(ClC2) = 69.5 Hz, 'J(C3C4) = 35.6 Hz. dFor the CH,C(O) fragment. 'Measured in the solid state. flJ(C2C3) = 66.0 Hz, 'J(C4CS) = 71.8 Hz.

5. ONE-BOND CC SPINSPIN COUPLING CONSTANTS IN DERIVATIVES OF BENZENE

As was shown for substituted acetylenes and ethylenes, the changes effected by substituents on 'J(CC) magnitudes dramatically exceed those caused by a change in the formal hybridization of the coupled nuclei. Also in substituted benzenes the influence of substituents upon 'J(CC) is very strong (Tables 19-23). The 'J(ClC2) coupling of 29.5Hz (in Et20)136 and 27.8Hz (in THF)138 was found for phenyllithium, which is the smallest one-bond CC coupling determined so far for derivatives of (Table 19). A somewhat larger 'J(CC) value of 34.7 Hz (in THF) and 36.1 Hz (in Et,O) was observed in phenylmagnesium bromide. 136~139The largest coupling of 82.3 Hz was found in 1,2-difluorosubstituted benzene across the C1C2

154

K . KAMIENSKA-TRELA

Table 17. One-bond CC spin-spin couplings across Csp2(arom)Csp3, Csp2(arom)Csp2and Csp2(arom)Csp bonds in derivatives of benzene; data for substituent at C1. All values are in hertz.

Substituents at carbon X c1 CH3 CH2N02 CH2N02 CH3

c2

c3

C4

C5

C6

NO2 NO2 H H

H H NO2 H

H H H CN

H H H H

H H H H

'J(Csp2(arom)Csp3) Ref. 44.1 50.1 47.4 43.4

132 132 132 133

'J(Csp2(arom)Csp2 CH=CHC02Ettrans

CHO CHO COOH COOH COOCH3 COOCH3 COOCH3 COOCH, COOCH, COOCH, COOCH3 COOCH3 COOCH3 COOCZHS COOC2HS COOCH3 N(CH312 COOCH, COOCH, COOCH3 COOCH, COOCH3 COOCH3 COOCH3 COOCH3 COOCH3 COOH COOH COF COF COF

H-

56

CH3

H

H H H NO2 H H H H H H H H H H H H H H H H H H H H H H H H H H H

H H H H H H H H H H H H H H H H H CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 H H H H H

52.0 50.5 73.6 74.3 74.5 75.0 74.8 74.7 74.7 75.6 75.7 76.4 77.2 77.1 77.9 78.1 78.5 73.4 74.1 73.7 74.3 74.6 74.7 75.8 76.4 76.7 71.9 72.0 80.6 80.8 81.6

93

132 132 132 132 134 134 134 134 134 134 134 134 134 127 127 134 134 134 134 134 134 134 134 134 134 134 135 135 135 135 135

155


Table 17-continued 'J( Csp2(arorn)Csp(N)) H H H H H H H H H H H

CN CN CN CN CN CN CN CN CN CN CN

H NO2 OCH3 H CH3 H

NO2 H H H H OCH3 H H H H CF3

c1

Br F CF3 H

H H H H H H H H H H H

H H H H H H H H H H H

83.9 82.0 83.7 80.3 81.5 80.6 84.0 83.1 83.0 80.8 82.6

136 136 133 133 133 133 133 133 133 133 133

-

'J( Csp2(arorn)Csp) Compound

'J

Ref.

Compound

Table 18. One-bond spin-spin couplings across ~ U ~ U - X - C & I ~ - C H ~ M ( CallHvalues ~ ) ~ ; in hertz.'37 Substituents

'J(CC)

X

M

Me0 Me0 Me0 Me0 Me Me Me Me

C Si Ge Sn C Si Ge Sn

36.0 41.5 43.2 43.5 36.0 41.1 42.7 43.0

'J

Ref.

Csp3Csp2(arorn) bonds

Substituents

'J(CC)

x

M

H H H H CN CN NO2

C Si Ge Sn Si Sn C

"43.8 Hz (measured in our laboratory, see Note added in proof). b42.6 Hz (measured in our laboratory, see Note added in proof).

36.0" 40.9 42.5 42.8 40.3 40.4 36.Ob

in

156

K. KAMIENSKA-TRELA

b ~ n d . ' ~ ~ Ne , ' ~xt' to it are the C1C2 couplings in l-fluoro-2-nitrobenzene, l-fluoro-2-methoxybenzene, 1-methoxy-2-nitrobenzene and l-fluoro-2bromobenzene, where the corresponding 'J(ClC2) values are 80.2 Hz, 79.7 Hz, 77.6 Hz and 77.4 Hz136 (Table 20). The influence of substituents in substituted benzenes is limited practically to the couplings across Cipso-Cortho bonds. This close-range character is typical of inductive effects which are assumed to be concerned with an appropriate redistribution of s-electron densities. This was observed for the first time by Wray et a/." for monosubstituted benzenes and by others for multisubstituted aromatic rings.' The most illustrative example of this phenomenon is provided by 4-fluorophenylmagnesium iodide'36 in which the effects produced by the substituents are of a similar magnitude but of the opposite sign, negative for the MgI substituent and positive for fluorine. As a consequence, three very different 'J(CC) values are found in this small molecule. These are 37.0 Hz, 52.0 Hz, and 70.4 Hz (see below and Table 21):

Q

37.0 Hz

52.0 Hz

70.4 Hz

F The results obtained for Li and MgBr (or MgI) derivatives extend considerably the region covered by 'J(CC)s (over 50 Hz), which allows one a reliable estimate of the influence of substituent electronegativity upon the parameter concerned. As can be seen from Fig. 5 , the relationship is evidently non-linear: 136 'J(CC)

=

30.57

+ 29.82 In Ex

(5) It should be stressed that no particular meaning can be assigned to this form of relationship; the important feature is its strong non-linearity . This contradicts some reports in the literature where the linear relationship for ' J ( CC) vs. Ex was suggested for benzene derivatives. 14c-142 H owever, the 'J(CC) range discussed in these references was rather narrow, i.e. of ca. 25 Hz only. It should also be mentioned that a correlation of 'J(CC) couplings with the bond polarity index in benzenes has been investigated by Reed and Allen,'43 but no meaningful relationship has been found by these authors.

157


Table 19. One-bond CC couplings in rnonosubstituted benzenes (in hertz).

~~

~

~

Compound Substituent Solvent Concentration 'J(ClC2) 'J(C2C3) 'J(C3C4) Ref. no.

'J(CC) in phenyllithiurn (27) and phenylrnagnesiurn bromide Et20 Saturated 29.5 27 Li 27.8 27 Li THF -a 28 MgBr Et20 1.00 M 36.1 28 MgBr Et20 2.00 M 35.3 28 MgBr THF 1.00 M 34.7 'J(CC) in silyl and boryl substituted benzenes (29-33) 49.5 H3Si 29 49.2 30 Ph2(H2C=CH)Si 31 Ph2(2'-CH3-c-C3H2)Si 48.8 32 Ph2(2'-C5Hll-c-C3H2)Si 48.6 33 BH2 48.4

(28) 51.4 50.3 52.0 52.3 51.9

55.8 56.0 56.0 56.0 55.9

136 138 136 136 136

54.7 54.7 54.8 55.0

55.4 55.6 55.4 55.4 -a

71 119 119 119 144

57.5

56.0

145

-a

'J(CC) in benzyl bromide, C6H5CH2Br(34)

34

58.6

aNot reported.

The low value of 'J(CC) in phenyllithium (29.5 Hz) was attributed by von Schleyer and c o - ~ o r k e r s ~ ~to* ,a' ~shortening ~ of the Cipso-Cortho bond accompanied by the contraction of the ips0 angle when compared with the remaining phenyl derivatives. The changes observed were interpreted by the authors in terms of re-hybridization which occurred at the carbon atom involved upon substitution. 6. ONE-BOND CC COUPLING CONSTANTS IN HETEROAROMATIC SYSTEMS

The data published for heteroaromatic systems during the period reviewed include a collection of the couplings for unsubstituted five-membered heteroaromatic ring systems measured by Witanowski and Biedrzycka14' (Table 24), a large set of 'J(CC) couplings measured by Gronovitz et al. 148,349 for thieno[c]quinolines and thieno[c]isoquinolines'48 (Table 25) (Table 26) and some new data for indole and dithien~[b,d]pyridines'~~ derivative^'^^^'^^ (Table 27).

158 1

N

I

K . KAMIENSKA-TRELA

75 7

1 1

N

0 I

65

7

-0 7

-0

c

55

0

n I

a,

6

45

35

25

I 0.5

I

I

I

I

I

I

1

1.o

1.5

2.0

2.5

3.0

3.5

4.0

Pa uli ng 's e l e c t r o n e ga t ivi t y of subs t it uen t Fig. 5. The 'J(CC) vs. Ex relationship in monosubstituted benzenes. The Pauling electronegativities Ex of the first atom of substituent are taken from R. McWeeny, Coulson's Valence, p. 163, Oxford University Press, London, 1979. (Reprinted with permission from ref. 136.)

Table 20. One-bond CC spin-spin coupling constants in ortho and meta disubstituted benzenes; substituent X at C1; all values in hertz.

Substituents

x ortho F F F OCH3 F OCH3 NO2 OH NO2 CH3 MgI

'J(CC)

Y

CIC2

C2C3

C3C4

F OCH3

82.3 79.7 80.2 77.6 77.4 75.2 73.0 71.8 67.6 67.0

72.3 67.5 67.4 66.3 64.6 66.2 60.7 67.4 58.7 68.2 54.4

57.2 57.7 56.8 57.8 56.0 55.0 54.7 58.6 55.9 56.9 56.3

NO2 NO2 Br Br I NO2 CHO NO2 CH3

-a

C4C5

C5C6

C6C1 Ref.

-

55.7 55.0 55.5 56.5 56.0 55.2 53.8 56.3

57.2 56.7 56.9 57.9 57.5 58.0 57.3 58.7 56.3

72.3 73.7 71.7 66.2 71.7 66.3 66.2 67.5 67.7

56.4

51.6

38.0

-'

-a

-a

136 136 136 136 136 136 136 132 132 132 136


159

Table Zkontinued Substituents X

J( CC) Y

NO2 CN NO2 NO2 Br Br I CH2N02

C1C2

C2C3

C3C4

C4C5

C5C6

C6C1

Ref.

73.5 70.1 70.1 69.4 67.0 68.8 66.5 68

69.9 62.4 70.1 69.4 66.9 65.1 62.6 56

67.2 59.5 67.8 68.0 64.3 63.2 60.1 56

57.0 57.1 55.6 56.8 55.6 55.0 54.5 56.6

58.3 56.0 55.6 58.4 58.7 57.2 56.6 62.1

71.0 67.6 67.8 66.9 67.5 68.1 67.6 68

136 136 132 136 136 132 136 132

“Not determined.

Table 21. One-bond CC spin-spin coupling constants in para-substituted benzenes; substituent X at C1; all values in hertz. Substituents

X

‘J(CC)

Y

F F F F OCH3 OCH3 NO2 NO2 COCH3 CH2N02 OCH3 OCH3 OCH3 OCH3 CH3 CH3 CH3 CH3 CN CN “Not determined.

ClC2/ClC6 72.5 72.1 70.9 70.4 68.0 66.2 67.9 68.3 58.1 59.1 67.6 67.6 67.6 67.6 56.9 57.3 57.3 57.3 60.3 60.8

C2C3/C5C6

C3C4K4C5

Ref.

-a

72.5 62.0 67.9 37.0 65.0 68.1 60.7 68.3 67.5 67.9 57.0 57.0 57.3 57.3 56.4 56.8 56.8 56.8 55.9 55.9

136 136 136 136 136 136 136 132 132 132 137 137 137 137 137 137 137 137 137 137

55.9 59.1 52.0 57.6 54.3 57.3 -

56.6 56.8 58.0 58.6 58.6 58.7 56.6 57.2 57.2 57.2 58.0 58.0

160

K. KAMIENSKA-TRELA

Table 22. One-bond CC spin-spin coupling constants in multisubstituted benzenes; all values are in hertz. ~~~~

~

Substituent at carbon

c1

C2

NO2 NO2 NO2 c1 NO2 COOH H NO2 OH NO2 H

CHO OH F

Coupled nuclei

C3

C4

C5

C6

C1C2 C2C3 C3C4 C4C5 C5C6 C6C1 Ref.

H H H H NO2 NO2 NO2

NO2 NO:, NO2 NO:, H H H

H H H H NO2 NO2 NO:,

H H H H H H H

69.3 70.6 70.6 68.9 72.4 72.1 72.1 66.2 80.6 71.1 72.1 67.8 -a 71.3 70.3 69.4 60.2 69.3 70.5 70.5 77.0 77.0 72.7 71.7 70.5 70.5 70.5 70.5

54.3 59.7 132 60.5 67.6 132 58.4 72.1 132 57.3 68.7 132 69.3 60.2 132 71.7 72.7 132 70.5 70.5 132

"Not determined.

Table 23. One-bond CC couplings in trans and cis '3C-labelled azobenzenes; all values are in hertz. 14'

q

N=N

NEN

db

3 43

(

trans

cis

'J(ClC2) ~

~

'J(C2C3)

'J(C3C4)

~~

[ 1,1'-13c2]trans [4,4'- I3C2]trans [2,2',6,6'-I3C4]trans 4,4'-dichlor0[4,4'-'~C:,] trans 4,4'-diiodo[4,4-13C2]trans

-

56

-

65 61 58

2,2'-dirnethoxy[4,4'-l3C2] trans [ I , I ' - ' ~ c ~cis ]

-

[4,4'-''C2] cis 4,4'-diiod0[4,4'-'~C~] cis

52 61

The compounds studied by Witanowski et al. 147 represent all relevant diazole, triazole, oxazole, oxadiazole, thiazole and thiadiazole systems. The couplings measured for them show a clear distinction between carbon-carbon bonds which are formally double and single, respectively. Those across the formally single C3-C4 bonds vary from 45.6 to 52.2Hz, whereas the couplings across the double bonds range from 58.9 to 70.7 Hz.


161

This is at variance with the situation in pyridine where both couplings in question are of almost the same value, 'J(C2C3) = 54.3Hz and 'J(C3C4) = 53.7 Hz.lS2It is of interest to note that these values lie almost in the centre of the gap between the two subranges mentioned above. The authors attempted to correlate the obtained 'J(CC) values with the corresponding bond lengths but in spite of some apparent trends no general correlation has emerged. This is in agreement with the observation made recently by Kamienska-Trela et al. lS3 for 9aH-quinolizine-l,2,3,4tetracarboxylate and some of its alkyl derivatives. A multivariate principal components data analysis has been applied by Gronovitz et al. 149 in order to estimate 'J(CC) in dithieno[b,d]pyridines from shift data, the azomethine C-H coupling constant and MNDO bond orders, and the calculated values have been compared with experimental ones. Good results have been obtained for the couplings of the pyridine subunit. It is of interest, however, that the last parameter gave only a minor contribution to the coupling variability.

7. ONE-BOND CC COUPLINGS IN SUBSTITUTED ALIPHATIC CYCLIC AND HETEROCYCLIC SYSTEMS

7.1. Three-membered ring compounds

One-bond CC spin-spin couplings in substituted aliphatic cyclic compounds deserve a special attention. Among them those in substituted cyclopropanes and cyclopropenes are of special interest. A set of coupling constants for a series of 1,l-dihalogeno-2-alkoxysubstitutedcyclopropanes has been recently reported by Krivdin et ~ 1 . ' and ~ the data for lithium-substituted compounds have been published by Seebach and co-workers'" (Table 28). The for 2,3,3-triphenylcouplings measured by Krivdin et cyclopropene-l-carboaldehyde represent the first set of 'J(CC) couplings measured for substituted cyclopropene (Table 29). The data for a few silyl-substituted cyclopropenes have been obtained by Jankowski et a/. l9 In all these compounds the couplings across the endo-cyclic bonds are substantially smaller, whereas the couplings across the exo-bonds are much greater than the corresponding couplings in analogously substituted openchain compounds. The 'J(CC) couplings collected for substituted cyclopropanes in Table 28 vary from 16.55 Hz in 1,l-dichlorocyclopropane (85),76 to < 0.5 Hz in l-phenyl-2-lithiocyclopropane(72).Is' Thus, the range covered by these couplings is quite large. However, out of the substituents involved only Li exerts the strong influence of ca. 12 Hz on the magnitude of 'J(CC) in these compounds. As a consequence, the value of 'J(CC) across the bond with the

'

162

K. KAMIENSKA-TRELA

Table 24. One-bond I3C-l3C coupling constants (in hertz) in five-membered heteroaromatic systems. 4

50 0 3

2

F 0 N

36

35

0

37

38

F N N

N

1h3

0

p

S

41

42

46

47

43

N S’

44

45

CH,

48

CH,

7-7 49

Compounds 35 Furan 36 1,2-0xazole (isoxazole) 37 1,3-oxazole

38 1,2,5-oxadiazole (furazan) 39 1-Me-pyrrole 40 1-Me-pyrazole 41 1-Me-imidazole 42 1-Me-1,2,5-triazole 43 l-Me-1,2,3-triazole 44 Thiophene 45 1,2-thiazole 46 1,3-thiazole 47 1,2,5-thiadiazole 48 1,2,3-thiadiazole 49 4,5-dimethylfuroxan 50 4,5-diethylfuroxan “Not determined.

50

IJ(3,4)

‘J(2,3)

Ref.

-a

69.1 67.7 70.7

9 147 147 146 9 147 147 147 147 9 147 147 147 147 154 154

48.7 -

45.6

-

51.5

66.3 64.6 66.5

-a -

51.6

-

-

64.5 64.2 62.2 61.1

-a

52.5 -

48.1

-

-

58.9

63.0 62.0


163

Table 25. One-bond CC spin-spin coupling constants (in hertz) in thieno[c]quinolines and thieno[c]isoquinolines. Reprinted from ref. 148 by permission of John Wiley & Sons Ltd.

51

54

Carbons coupled

Cl,C9b c1,a C2,C3 C3,C3a C3a,C9b C3a,C4 C5,CSa C5a,C9a C5a,C6 C6,C7 C7,C8 C8,C9 C9,C9a C9a,C9b

53

52

55

56

Positions of nitrogen and sulphur atoms (compound no.) N5,S3

N5,S2

N5,Sl

N4,S3

N4,S2

N4,Sl

58.9 65.3

67.7

-

60.6 64.6

68.8

-

-

55.1 57.9 -

55.1 65.5 58.7 53.4 59.9 58.4" 57.6

-

65.2 52.1 52.9

-

56.3 66.8 57.4 54.5 58.8 58.5" 57.1

"Calculated from the inner lines only.

65.9 58.4 54.8 54.9 -

54.5 65.7 58.8 53.3 60.4 57.7 60.9

-

59.7

76.8 55.3

66.6 67.3 58.3

-

-

55.0 53.7 56.9 59.6 53.5 59.7 56.7 58.1

52.3 54.5 58.1 58.2 54.6 58.6 58.6 57.3

54.4 53.9 57.1 59.6 53.5 60.1 57.7 61.6

-

-

164

K. KAMIENSKA-TRELA

Table 26. One-bond CC couplings in dithieno[b,d]pyridines (in hertz).'49

59

58

57

62

61

60

63

rn

64

65

Position of S atoms (compound no.)

Coupling

c1c2 ClC8b C2C3 C3C3a C3aC8b C5C2a C5aC6 C5aC8a C6C7 C7C8 C8C8a C8aC8b

1,6

3,6

1,8

3,8

2,6

1,7

2,8

3,7

2,7

(57)

(58)

(59)

(60)

(61)

(62)

(63)

(64)

(65)

-

64.8 60.2

-

65.3 61.1

-

-

-

-

68.5

-

69.9

64.3 61.1

60.9

-

67.9 67.2 56.1 61.2

57.0 62.2

54.1

57.5

-

-

65.8 59.2 69.9

-

-

-

65.5 58.3 58.2

68.2 62.2 55.2 58.1 58.2 53.0 66.8

57.5 58.7 58.3 53.7 66.9

-

-

-

-

-

-

68.5

69.9

"Calculated from the inner lines only

-

76.4 54.4 58.5

-

56.5

-

64.6 59.4 59.7

66.6 67.9 59.5 55.0 64.6 53.3"

-

68.7 63.8

-

76.5 53.9 55.1 58.7 56.3 67.7 -

63.2

-

61.1 55.7 64.8 51.6

-

77.5 54.7 53.7 65.6 -

-

-

67.3 60.3

68.8 59.9


165

Table 27. (a) One-bond CC spin-spin couplings (in hertz) in dimethyl l-acetyl-6hydroxy-4,5-indoledicarboxylate(66).150 COOCH, I

COCH,

'J(C2C3)

'J(C3C9)

'J(C5C6)

'J(C6C7)

'J(C7C8)

'J(C8C9)

71.5

55.9

69.5

67.7

62.4

54.3

'J(C4C9) 53.5

'J(C4COO) 'J(C5COO) 74.4

75.1

(b) One-bond CC spin-spin couplings (in hertz) in tryptophane (67).15' COOH

H H O

'J(C2C3)

'J(C3C9)

'J(C3C10)

70.3

55.8

48.4

lithium substituent attached is close to zero (< 0.5 Hz). An influence of the remaining substituents, including the strongly electronegative ones, such as OAlk, CI and Br, is of few hertz only, and clearly non-additive. Small 'J(CC) values in derivatives of cyclopropane and their rather weak and non-typical sensitivity towards substitution can be explained in terms of small contribution of the s electrons to the orbitals forming a CC bond in the cyclopropane ring. This explanation is in accord with the theoretical results of Eckert-Maksic and M a k ~ i c who l ~ ~ calculated the following hybridization indices for unsubstituted cyclopropane:

n

C-C 3.69-3.69; C-H 2.49

166

K. KAMIENSKA-TRELA

Table 28. One-bond CC coupling constants in substituted cyclopropanes: all values are in hertz. 3

Substituents Compound no. 68 69

X

Y

H H Sesquiterpene globulol"

70 71 72 73 74 75 76 77 78 79

H C6H5 C6HS COOH H C6H5 C(CH,)=NOH C6HS

Z H

CH3 Br H H COOH Br H Br H

OC2Hs

c1

CH3 Li Li H COOH H H Br H

80

O(CH&CH3

Cl

c1

81

OCH(CH3)z

C1

c1

82

O(CH&CH3

C1

c1

c1

c1

83

c1

84

c1

85

H

c1

c1

Coupled nuclei

'J(CC)

Ref.

12.4 14.6 14.6 15.8 'J(C5C5'ax). The 'J(C5C5') constant of 35.9 Hz found for isomer 143b provided evidence that this compound exists as a mixture of two conformers: cis(E) (2eq, 5ax) and cis(E) (2ax, 5eq). A presence of small amount of an imine form (143d) has also been postulated by the authors in order to explain 143a2143b 3 1 4 3 ~interconversion.

143 b

143 c

Reprinted with permission from ref. 183 An influence of the nitrogen lone pair of ca. 6-8 Hz on the corresponding 'J(CC) couplings has been also reported for numerous aza-aromatic compounds including a-picolines, pyridazines, pyrazines and pyrimidines. The relevant data have been collected in Krivdin and Kalabin's review.' The in order to lone pair effect has also been invoked by Afonin et rationalize the results obtained for a large series of vinylazoles, where the 'J(CC) couplings across the double bond of the vinyl group have been measured. Some of these compounds exist as the mixtures of s-trans and s-cis conformers with respect to the N(2) nitrogen, and the equilibrium shifts towards s-cis conformer upon alkyl substitution in position 5. This is


185

Table 44. Lone pair contribution, ZIP,to 'J(CC) cis" and 'J(CC) trans' in monoprotonated (144a) and non-protonated acetone (144b) and in non-protonated acetoxime (134).

"

Compound lUab

144b' 134

'J(CC) cis

'J(CC) trans

1.28 1.54 2.48

-1.03 -0.99 -2.14

A(&

-

trans)

2.31 2.53 4.62

Exp. 4.5

-

7.8

9,,stands for the sum of all J,o,,b contributions in equation (6), where i or j corresponds to the unprotonated oxygen lone pair in monoprotonated acetone or to the nitrogen lone pair in acetoxime. bCorresponds to the 6-31G**/MP2 optimized geometry of protonated acetone. 'Corresponds to the 6-31G**/MP2 optimized geometry of unprotonated acetone with the 0 - H - length and bond angle taken from the standard model of Pople and Gordon.

accompanied by a small increase (ca. 2Hz) in the 'J(CC) value. Thus, for example the coupling across the vinyl bond, 'J(CC) = 77.7 Hz was found in 1-vinylpyrazole and 79.7 Hz in l-vinyl-3,5-dimethylpyrazole.

s - trans

s

- cis

The influence of the oxygen lone pairs on 'J(CC) is also substantial. In an interesting study by Krivdin et aZ.ls5 on the effects of protonation on acetone, a difference of ca. 5 Hz was observed between the coupling across the bond cis arranged to the oxygen lone pair and the trans one, the cis coupling being the greater one (Table 44).

cis

'J(CC) = 40.0 Hz

CH3

CH3

\C/

II

O+

\

H

'J(CC) in acetone = 40.5Hz

trans 35.5Hz

186

K. KAMIENSKA-TRELA

Calculations performed by the CLOPPA (Contributions for Localized Orbitals within the Polarized Propagator Approach) method allowed the authors to rationalize the results obtained in terms of the lone pair effect, though the other effects, such as the net positive charge effect and the geometry effect, have been discussed by them. In particular, a substantial positive charge which appears on the carbonyl carbon atom upon protonation of one oxygen lone pair should yield a reduction of both 'J(CC) cis and 'J(CC) trans couplings at the rate of 1.5-2Hz per O.le of positive charge. The calculations have been performed by the use of equation ( 6 ) , where the indirect spin-spin coupling constant is presented as a sum of MO contributions, each of them depending on, at most, two occupied and two vacant localized molecular orbitals (LMOs) (Table 44):

The above results are in agreement with the results of the earlier experimental studies performed by Krivdin et u Z . ~ on 'J(CC) couplings in a large series of vinyl and phenyl ethers, sulphides and selenides. Also the theoretical studies performed by the same group of authors for 'J(CC) in H2C=CHR and CH3CH2R where R = OH, NH2, SH and PH2,1g6and for protonated and non-protonated furane and some methyl-substituted furanesIg7 corroborate the conclusion that an influence of the lone pair has always to be taken into account.

9. ONE-BOND CC COUPLINGS IN STRUCTURAL STUDIES OF COMPLEXES

One-bond CC couplings have been measured by many authors for a variety of complexes with a hope that they should provide an insight into the structure of this most interesting group of compounds. Indeed, the changes occurring in 'J(CC) couplings upon complexation are often very strong and are the source of valuable information on the electron re-distribution which takes place within a carbon-carbon bond upon coordination of the ligand to the metal. One of the most intriguing results obtained in this field concerns the couplings measured by Chisholm et al. ,1883189 for the alkynes bonded to hexaalkoxides of dimolybdenum and ditungsten. The couplings range from 10.3 Hz in W2(0-t-Bu)6(py)(p-C2H2) to 43.4Hz in CP~MO~(CO)~(~-C (Table ~ H ~45). ) ~ *A~coupling of 55.9 Hz was observed by Aime et in C O ~ ( C O ) ~ ( ~ - C ~These H ~ ) .are the smallest values reported so far for the couplings across the triple CC bond. They are accompanied by the low 'J(CH) coupling of 192 Hz observed by Chisholm in


187

Table 45. One-bond CC coupling constants (in hertz) in M ~ ( O R ) ~ ( P Y ) ~ ( P - C ~ H ~ ) complexes and related carbonyl derivatives, with the CC distances, rcc. Compound

Type

'J(CC)

rcc(A)

Ref.

I

28.2 26.9 23.2 15.8 11.4 19.0 10.3 55.9 43.4

1.368(6)

189 189 189 188 189 189 189 190 189

I I I1 I11 P2

112

-

-

-

1.39(2) 1.39(2) 1.44(1) 1.327(6) 1.337(5)

W2(0-t-Bu)6(CO)(p-C2H2) (lJ(CH) in ethyne = 249 Hz);lS8 both coupling constants have been invoked as evidence for the dimetallatetrahedrane paradigm for the compounds under study. A rough inverse correlation between 'J(CC) values and the corresponding rcc distances has been noted by the authors for these complexes. However, since the carbon-carbon distances in complexes are measured with rather poor rsd the relationship mentioned above should be, as the authors themselves emphasized, treated l ~ ~ with great caution. It is also rather surprising that Chisholm et ~ 1 . have found that 'J(CC)s in the compounds studied by them are proportional to AE = &(LUMO)- &(HOMO). According to McConnell's average energy approximation all contributions to the couplings are inversely proportional

I R

R I

188

K. KAMIENSKA-TRELA

to the average energy AE.32 However, in spite of some inconsistencies in theoretical interpretation, Chisholm's results are certainly very interesting and shed new light on the electronic structure of alkyne complexes. A new, rather unusual, type of compound, 144, in which a diorganotin dication is stabilized by n- coordination to two C=C bonds has been reported by Wrackmeyer et ~ 1 . lAn ~ ~interaction between the Sn2+ cation and the triple bond is reflected by the substantial decrease in the 'J(CC) coupling across this bond (101.0 Hz only). For a comparison 'J(CC) of 119.2 Hz was found in sodium triethyl-1-propynylborate[CH3C=CB(C2H5)3]- Na+.

Quite a number of papers have been devoted to various complexes of olefins (Tables 46-50). Philipsborn and c o - ~ o r k e r s ' ~ 'ha - ~ve ~ ~reported the 'J(CC) data for a series of q4-diene, q3-allyl and q2-ene complexes of Fe, Ru and 0 s ; Benn and R ~ f i n s k a ' ~have ~ ' ~measured ~ the 'J(CC) couplings for a series of q2-olefin-Ni(I1) complexes; and Yamamoto et ~ 1 . lhave ~ ~ published the 'J(CC) values for (q-pentamethylcyclopentadienyl) titanium-diene complexes. The 'J(CC) couplings for olefin complexes of Rh(1) were published by Fitch et ~ 1 . Further ~ ' ~ examples include the data for various complexes of unsubstituted ethylene studied by Huffman and c o - ~ o r k e r s , ' ~ *Bender ~ ' ~ ~ et al. ,200 and others2"',202 (Table 50). Finally, 'J(CC) couplings in complexes of heterocyclic dienes and ethenes have been measured by Wrackmeyer and c o - w ~ r k e r s . ' ~ ~ ~ ~ ~ , ~ ~ ~ In all cases the 'J(CC) values are significantly smaller (by ca. 20 to 30 Hz) than the coupling constants in a free ligand. This remains in striking contrast to the behaviour of one-bond hydrogen-carbon coupling constants whose values slightly (by ca. 2Hz) increase upon complexation and lie between those in ethylene ('J(CH) = 157 Hz) and cyclopropane ('J(CH) = 160 Hz). The large decrease in 'J(CC) occurring upon complexation has been invoked by many authors as evidence that the complexes are best described as metallacyclopropanes (IV) rather than n--complexes (V). Thus, for example, the osmacyclopropane structure V has been assigned to the O S ( C O ) ~ ( C ~ H ~ ) complex by Bender ef ~ 1 . ~ " The magnitude of the decrease in 'J(CC) upon complexation varies upon the nature of the metal involved and depends on the geometry of a given complex, but remains constant within a given series.


189

Table 46. The typical 'J(CC) couplings in the s-cis complexes of butadiene; all values in hertz.

Fe

Co

Ru

0s

W

Th

Zr

Hf

43.9

43.5

42.4

38.8

38.5

38.0

37.4

34.2

The following trend emerges when the 'J(ClC2) data available for the s-cis complexes of butadiene are compared (Tables 46 and 47):

Fe = Co > Ru > 0 s = W = Th = Z r > Hf As far as the geometry of the complex is concerned, the 'J(ClC2) coupling reported for (q4-s-truns-C4H6)ZrCp2 is considerably greater than that for (v4-s-cis-C4H6)ZrCp2 complex, 45.9 Hz and 38.6 Hz, respectively. Furthermore, the C1C2 coupling in the endo titanium-diene complex is greater (ca. 43 Hz) than in the corresponding ex0 complex (39.1 Hz), while

. M
OK CI 9H2406 Tagitinin C 160.5 129.6196.9 138.8137.1 76.1 47.1 74.1 48.4 71.9 136.1 169.8 124.4 28.9 19.7 C 50 C22H2607 Germ~cra-1E,4Z,11(13)-trien-lZ,6a-olide,l0a-acetoxy-8fl-cpoxyangeloyloxy-3-~1~11 157.7 128.8 195.9 139.2 135.8 75.6 47.5 73.9 47.5 79.6 135.8 170.0 124.9 24.6 200 C 231 C24H2809 Niveusin A. diacetate 159.6 128.6 193.8 137.4 141.1 75.0 47.6 72.5 47.3 79.5 135.4 170.0’ 125.2 24.4 64.1 C 404

CARBON-13 NMR SPECTRA OF SESQUITERPENE LACTONES

303

Table ll.--continued Mol. formula C-1 C-2 C20H2606 131.9 130.5 C20H2806 35.6 29.8 C20H2807 79.2 39.1 C22H3008 76.2 35.5 C24H3209 73.8 34.8 75.1 33.5

No. 279 280 281 282 283

a b

Name I Chemical shifts C-3

C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-12 C-13 C-14 C-15 Sol. I k l ' Tifruticin, deoxy 72.9 146.8 125.9 73.9 51.3 68.9 43.8 70.8 135.5 169.4 122.6 29.5 25.1 C 2x9 Germacr~4~11(13)-dien-l2,~-olide, 8p-angeloyloxy-3p,10-dihydraxy 75.0 138.4 127.5 74.2 48.1 74.4 38.9 71.7 136.7 170.6 123.5 31.5 23.9 C 27') Germacra4Z,11(13)-dien-l2,~-olide,8~-angeloyloxy-l~,3~,l~-trihydroxy 72.2 139.8 127.3 74.2 48.7 73.1 39.5 73.1 137.8 170.0 122.5 24.2 24.0 I' 219 GermacradZ,ll(l3)-dien.12,6a-olide, 3-aeetoxy-8p-angeloyloxy-l~,lOa-dihydriix~ 68.6 144.1 125.6 74.6 49.3 70.2 38.4 72.3 135.8 169.7 123.1 27.7 23.5 C 279

Germ~er~4~11(13)-dien-l~6a~lide, lp,3p-diacetoxy-8p-angeloyloxy-lOa-hydri1x~ C C

72.4 135.2 128.5 74.0 48.0 73.2 39.1 72.7 135.8 170.6 124.0 24.8 24.5 69.8 140.8 125.8 75.1 47.1 70.9 38.5 72.6 136.3 169.7 123.6 29.3 22.5

279

279

Other carbons: 254 Mebu: 175.6 41.3 26.5 11.5 16.6; 255 Mebu: 175.7 41.2 26.5 11.6 16.7 #Me: 50.4: 256 Mac: 166.1 135.7 126.9 18.1; 257 Epang: 168.6 59.7 60.2 13.2 19.0; 258 Ang: 166.1 126.7 14O.'J 15.8 20.5; 259 Mac: 166.7 135.6 126.3 17.9; 260 Ang: 167.0 126.5 140.5 15.6 19.9; 261 Any: 165.7 126.5 141.0 15.7 19.4'; M 2 a M o c : 165.9 135.2 127.3 18.1; 262b Mac: 165.4 134.5 127.4 18.2. 263 i / t l f f , 175.4 21.1 18.8 18.4; 264 Tig: 166.4 127.3 139.8 14.6 11.8; 26% Mebu: 174.9 41.0 26.2 11.4 16.3. 265b Mebri: 175.0 41.0 26.5 11.4 16.0 266 iVal: 171.4 42.7 25.2 22.2 22.1; 267a Ang: 165.4 126.7 139.3 14.7 20.4; 267b Ang: 169.5 130.6 143.7 19.3 24.8; 267c Ang: 166.0 126.5 141.7 15.9 21.3: 267d Ang: 165.9 126.5 141.1 15.7 20.0; 268 Ang: 165.8 126.4 141.2 15.7 19.9: 270 OMe: a; 271 Glc: 95.7 74.2 78.9 71.2 78.5 62.4: 272 Mebu: 174.8 41.5 26.3 11.6 16.4 Ac: 169.2 21.0; 273 Mebu: 175.9 42.7 27.4 10.8 17.7 Ac: 170.9 21.7; 274 Ang: 166.1 126.6 140.9 15.5 20.4 Act 169.9 21.1: 27% Ang: I664 126.7 140.4 15.7 20.1; 275b Ang: 166.5 126.8 137.4 15.8 20.1; 276 Bur: 176.2 34.1 18.8 18.6: 277 Epang: 168.6 59.4 60.3 13.3 18.9 Ac: 169.2 21.7; 278 Ang: 166.2 126.6 137.7 15.8 21.6 Ac: 169 2b 20.1' 170.8' 20.8'; 279 Ang: 167.3 127.1 140.7 15.9 20.4; 280Ang: 167.1 127.3 139.3 15.7 20.3 2x1 A f f , ~ . 166.4 127.9 138.1 15.6 20.4; 281 Ang: 166.8 127.0 140.1 15.7 20.3 Ac: 171.4 21.2: 2x2 Aw: I66.j n OMebu HO"'

RO"

O'CO

254

R = H

255

R =

CH,

& 0

256

R = Mac

257

R = Epang

258

0

259

R = a-OMac

260

R = a-OAng

261

R = P-OAng

COOR

O'CO-

hOh&

H

262

R = Mac

267

R =

263

R = #But

268

R = TAC

264

R = Tig

265

R = Mebu

266

R = iVai

269

R = H

R'

R'

270

R = CH,

272

Mebu

H

271

R = Glc

273

Mebu

OH

274

Ang

OH

HO R'

R'

R'

R'

275

OH

Ang

276

H

!But

277

H

Epang Ac

278

R'

280

8-OH

H

H

281

@-OH

OH

H

282

OAc

Oh

283

OAC

OAc

279

304

M. BUDESINSKY AND D. SAMAN

Table ll.--corttinued No.

284 285

286 281

288 289 290 291 292 293 294

Mol. formula C-l C-2 C23H3207 75.8 32.6 C20H3006 35 3 29.5 C17H2006 73.2 23.2 C22H2808 39.5 32.5

Name I Chemical shifts C-4 C-5 C-6 C-7

C-10 C-ll C-12 C-13 C-14 C-15 %I I
OAc 577

580

579

578

n

581

582 583

,do, co OAc i OMebu

R'

ti

Anq

oco 584

HO,,,,

R'

AC

585

Mac

,,,% 0,

0

586

587

R = Mebu

588

R = Anq

R'

R=

589

But

O-OAng

590

Anq

p-OAnq

591

Sen

a-OAng

592

Ang

6-OMebu

593

327

328


Table 14. Carbon-13 chemical shifts of germacran-12,8-olides (type 11-methyl). Mol formula Name I Chemicalshifts C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-I2 C-13 C-14 C-15 Sol llrt 594 CI7H2404 ~ 3 S , 7 ~ 8 ~ l l R ) - G e ~ a c r a - l ( l O ) ~ 4 ~ i e n - 3-acetoxy l2,8-ol~~ c 231 123.9 29.1b 76.8' 133.44 127.4 30.7' 48.8 76.6' 43.3 133.1' 37.6 178.5 13.2 16.0 23.8 595 C17H2405 ( l S , 3 S , 7 q 8 ~ 1 O q l l R ) - G e m P e r - 4 Z ~ n - l 2 , 8 ~ lfacetoxy-1,lO-epoxy id~ c 237 62.9 28.3b 74.1 134.8 126.2 30.3b 48.5 78.3 40.8 57.8 37.3 177.8 13.1 17.5 23.8 5Y6 CISH2203 Laurenobidide, dihydro, desacetyl (a: average s uum at 115°C; b-c spectra oEconformers at 2 0 " ~ ) c 635 a 129.3 24.4 38.1 135.5 131.7 71.5 40.7 80.3 46.9 130.9 58.5 178.6 17.7 17.7 16.6 c 635 b 130.5 25.0 38.5 135.1 132.6 70.8 41.1 80.0 47.7 130.7 59.1 179.6 17.7 16.6 16.6 c 635 c 125.4 22.8 36.2 135.8 129.4 72.3 40.5 83.7 43.6 132.3 56.2 179.6 17.7 21.1 16.6 Laurenobiolide, dihydro (a: averages c m at 115°C; 597 C17H2404 tx:specma ope2 conformers at 2 0 " ~ ) a 129 7 24.8 38.4 138.7 127.7 73.9 40.7 80.2 47.2 130.9 56.8 177.1 17.1 17.1b 17Sb C 635 h 130.5 25.1 38.7 138.3 128.3 73.1 40.8 79.4 47.7 130.7 57.0 177.4 17.2 16.6 16.9 c 635 c 035 c 125.6b 23.1 36.3 139.3 125.1b 75.0 40.1 82.7 43.6 131.9 54.2 177.4 17.2 17.3' 16.9' ll~H-Cermacra-1(1O)E,4E-dien-lZ,8a-olide, 3a,6adihydroxy 598 C15H2204 c 302 126.6 32.8 74.0 137.6 129.2 70.7 59.4 80.0 48.0 132.0 41.1 179.5 17.9 16.2 15.8 599 C17H2405 ChamSonin, 3-ncetyl-ll~,l3-dihydro c 234 126.0 30.5 76.0 134.2 130.2 70.4 59.1 79.5 47.7 133.6 41.2 179.2 17.7b 15.7b 16.8 600 C17H2406 Artemisiifolin, Q-O-acetyl-14-hydroxy-llp,l3-dihydro 135.0 26.8 35.2 144.1 130.7 78.1 59.1 74.8 46.7 136.0 41.4 180.9 17.2 60.1b 61.0b M 1 5 0 601 C18H2405N2 Chamissonin, 3-acetyl, pyrazoiine C 215 126.4 30.8 76.2 135.1 130.1 65.5 60.4 80.1 47.3 134.0 100.7 172.7 25.8 15.8 16.X 602 C25H3208N4 no name c 263 129.0 28.1b a a 129.0 30.9b 52.4 83.6 a 133.4 99.4' 171.8 23.2' 67.7 64.3 603 C21H3209 (11R)-Tatridin B, 11,13-dihydro, 6-0-p-D-glucosyl hl 574 72.6 32.1 34.5 140.9 128.0 76.9 42.0 81.9 42.5 148.0 56.9 181.5 18.2 114.5 17.2 604 CISHIBOS Pertilie, j-epi, 11~,13-dihydro-4f%,Sa-epoxide c* 465 1370 27.4 77.1 61.8 58.9 28.4 38.6 79.5 39.3 131.5 37.1 178.7 12.9 1649 17.6 605 CISHI805 Pertilide, 3-epi, Il~,lJ-dihydro-4f%,5-epoxide c* 465 137.0 28.2 76.9 61.7 58.4 32.2 42.6 78.2 39.5 131.2 45.1 176.9 15.4 1647 17.3 606 ClSN2203 (4S,7~8~11R)-Germacr-1(1O)Esn-12,8-olidc, 3-OX0 c' 237 124.4 44.8 209.1 47.2 31.2 24.2 47.8 76.1 42.7 133.3 38.3 178.3 13.2 16.X I4 5 607 C17H2007 Scandenolide, dihydro I> I40 59.1 29.4 67.4 134.3 144.6 80.0 54.6 78.2 43.2 57.1 40.5 175.0 13.6 20.4 169.4* 608 CISli1606 Mikanolide, dihydro L I40 58.7 55.4 51.2 131.5 146.5 81.5 54.2 77.6 43.5 57.7 42.4 175.1 13.8 21.5 170.5 609 C151i2203 Nunolide, &pi, 11~,13dihydro-Q,5~-epoxy C'' 639 128.8 24.8 29.9 66.4 61.9 38.0 51.4 83.9 43.0 129.3 46.9 177 3 16.6 13.6 17 5 610 C15112204 Ivaxillin C' 308 65.1* 24.0 35.7 61.0 64.4b 26.3 45.6 82.0 45.1 57.7 40.6 177.9 11.5 18.2 163 611 ClSH1406 Liacine, neo s 476 87.8 a a a 60.2 71.1 148.P a 122.3 132.9 147.2b 11.9 1690 17.2 X5 612 CISHI405 ' LinderanineB c 6x2 131.2 22.2 24.5 61.4 65.2 72.9 146.5 140.8 119.5 134.6b 13I.Ob 167.9 9.8 16.6 168.8 613 CISHI605 Linderanine C c 682 129.2 22.5 24.8 60.9 64.4 75.1 155.5 79.6 48.2 130.7b130.6b 169.1 9.4 16.2 171 4 614 C21N2608 Rolandrolide, is0 c 2x5 51.2 68.8 133.1 135.3 129.3 150.5 143.7 69.9 44.3 72.6 125.6 169.1 54.4 2x7 20.1 615 C23H3008 Rolandrolide, iso, ethoxy c 2x5 51 2 68.9 133.0 135.5 129.4 152.0 143.9 70.1 44.2 72.6 123.6 168.8 61.8 28.7 2 0 2 616 ClSH1606 Linderanine A s (1x2 129.7 22.2 24.2 60.1 64.1 75.3 153.4 110.9 50.9 131Sb130.gb169.2 8.8 17.2 I704 617 CISHI806 Linderanine D c' 682 125.5 22.4 25.3 61.5 63.9 72.2 152.0 80.7 40.0 135.6b131.3b 169.0 9.5 19.9 171.6 61R C17H1806 Zeylaninone 68.7 21.9 20.8 129.9 149.8 74.3 152.7 75.8 128.7 137.4 130.8 171.6 9.2 17.3 172.4 C 3'13

No


329

Table M--continued (kher ciubons : 5% Ac: 169.7 20.8; 595 Ac: 169.4 20.7; 5971 Ac: 169.2 20.8; 597b Ac: 169.2 20.9; 597c Ac: 169.2 20.9. 599 Ar: 170.5 21.3; 600 Ac: 171.7 21.1; 601 C16: 79.8 Act 170.3 21.3; 602 C16-C21: 79.0 167.8 97.5' 23.5' 64.6 78.5 2xAc: 170.1 170.7 20.9 20.6; 603 Glc: 99.7 74.8 77.@ 71.7 78.3b 62.8; 607 A(. 1696b 21.1; 614 Muc: 166.3 135.3 127.4 18.4 Acr 170.4 21.2; 615 Muc: 166.1 135.3 127.0 18.4 Ac: 170.2 15.1 OEI: 66.7 21.2; 618 Ac: 169.4 20.4

AcO

AcO

594

p -

R'x''

595

R'

R'

H

OH

597

H

OAc

598

OH

OH

599

OAc OH

596

CH,OAc

600

R = CH,OH

604

R = o-CH,

605

R = P-CH,

HO

I

AcO""

601

606

603

607

608

609

9 q CO-6

610

611

612

OH

HO 614

R = H

615

R = Et

616

20-6

ec0 613

co--0 617

618

330


Table 14.--continued .-

No.

Mol. formula C-l C-2

619 C17H1807 70.2 23.3 620 CISH1606 64.1 22.3 621 ClSH1606 62.9 24.4 622 CI5H2005 62.2 25.5 623 CI6H2604 50.8 25.0 624 C16H2605 51.6 24.5

C-3

Name I Chemical shifts C-4 C-5 C-6 C-7

C-8

C-9 C-10 C-lI C-12 C-13 C-14 C-I5

Acutotrinol 21.0 130.3 149.3 74.7 148.9 105.7 129.3 138.9 133.4 169.7 Acutotrine 23.3 60.8 64.6 76.3 155.0 78.7 45.5 57.6 131.0 170.4 Acutotrinone a 58.9 63.4 71.9 151.7 76.5 124.2 145.8 132.1 170.9 Glechoman-12,8a-olide, Ip,l0a.4cr,Sp-diepoxy-8P-hydroxy 23.9 61.1 67.0 35.9 158.5 107.5 50.0 58.6 130.0 171.0 Glechomanolide, Ip,lOa-epoxy-4-methoxy 37.3 81.6 32.6 24.5 160.6 80.5 49.6 77.3 122.1 175.1 Glechomanolide, Ip,lOa-epoxy-8-hydroxy-4-methoxy 38.4 82.0 37.0 24.1160.1105.6 49.8 77.1122.9173.3

__

Sill 11c.1

9.1

17.8 172.5

C

303

9.3

17.0 171.7

A

393

8.9

17.1 173.2

A

393

8.8

17.1 18.9

A

2IX

8 0 25.4 2 5 3

I)Glc: 103.5 73.9 88.7 69.9 77.7 62.5 105.7 75.4 78.4 71.6 78.0 62.5: 1073 (;Ic-Z-UCinn-J',4'-OH: 98.5 76.1 74.9 71.8 78.2 62.6 126.7 114.9 148.2 147.3 116.3 122.1 166.4 11.5 6 145.9. 1074 Glc(3-->I)Glc(4-->I)G/c:103.5 74.1 88.5 69.8 77.8 62.5 105.4 74.7 76.3 80.9 76.6 61.X 104.8 75.1 78.3 71.5 78.1 62.5; 1075 Glc: 103.5 75.9' 75.6' 72.9 75.4' 62.1 Phac-4'-OH: 171 6 40.X 125.1 130.8(2) 116.2(2) 157.8; 1076 Glc: 104.0 75.1 78.2 71.6 75.1 64.9 Phoc4'-OH: 171.9 40.6 125.2 130.8(2) 116.2(2) 157.8; 1079 Ang: 166.7 127.0 139.7 15.7 20.3; 1080 ;Bur-3-OH: 175.1 42.4 6 4 4 13 5 : 10R1 inirr-2.3-OH: 169.2 77.2 68.1 21.9; lOE2a Mac-ep: 168.9 53.8 52.8 17.4; 1082b Mac-ey: 174.8 7 6 0 hX.l 21.6

1061

1065

1078

R = H

1079

R = Ang

1080

R = But-3-OH

1081

R = iEut-2.3-OH

1082

R = Mac-ep

1062

1063

1064

1066

R = H

1068

R = H

1067

R = Ac

1069

R = Ac

1077

1070

R = B-Glc

1071

R = iVal

1072

R = B-Gk

1073

R = B-Glc-2-OCinn-3'.4'-OH

1074

R = 8-Glc

1075

R = b-Glc-4-OPhoc-4-OH

1076

R = B-Glc-6-OPhoc-4'-OH

' 2 6-Glc

'-

B-Ck

'-

P-Glc

370

M. BUDESINSKY A N D D. SAMAN

Table 21.+ontinued No.

1083 1OX4

1085 a

h c

10x6 10x7 1088 1089 1090

1091

1092 Illy3 d

h c

1094 1OYs 1096 1097 1098 1099 1100

1101 1102

1103 1104

110s 1106

1107 I108

Mol formula Name / Chemical shifts C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 CY C-10 C-ll C-12 C-13 C-14 C-15 Sol Ref C19HZ 706CI Linichlorin B 5 1 7 39.1 73.7 152.0 45.6 78.1 47.3 75.9 35.8 141.4 137.5 168.8 122.1 118.7 114.1 c‘ 126 C22112308NC:14 Linichlorin B + TAI C 126 i? 4 36.0 76.9 149.5 45.9 78.0 47.6 76.1 35.2 140.4 137.1 168.6 122.8 IIX.6 119.5 C21H2809 Crepiside E M 320 47 5 38.9 80.2 150.2 53.8 81.0 50.2 73.9 42.2 144.9 141.1 173.0 122.4 117.3 115.7 P 450 46.0 38.4 80.5 150.0 51.2b 79.0 52.0’ 72.2 43.0 144.4 140.5 170.0 121.6 116.2 114.3 P 449 45.9 38.4 80.5 150.0 52.0b 78.9 51.2b 72.2 42.9 144.3 140.4 170.0 121.7 116.2 114.3 Crepiside G C29H34011 I’ 449 46 3 38.4 80.2 149.5 52.4 78.6 47.3 74.6 36.9 143.2 139.0 169.1 121.2 117.5 115 1 Crepiside H C29H34011 I’ 440 46.1 38.5 X0.9 149.8 52.1 79.0 51.3 72.2 43.1 144.5 140.5 170.0 121.7 116.3 114.5 C20H2606 (lS,3S,5~6R,7~XS)-Guaia~(l5),1q14),11(13)-trien-l2,6-olide,3-acetuxy-X-angeloyliix) 45.7 36.4 74.7147.2 51.7 78.1 47.8 73.4 37.4141.5137.6170.7122.4118.3116.0 C I9 C23K3009 Grnssheimin, 4,15-dehydro-JaH-dihydro-3-acetyl-8-(2‘-hydroxy-3’-acetoxyisohutyrdte) 4S3 365’ 75.3 147.1 51.7 77.5 47.4 74.8 36.2b141.1 137.5 173.4 121.9 118.5 116.0 C 46X CZYli34011 Grossheimin, 4,15-dehydro-3aH-dihydro-3-acctyl-8-(2’,3’-diacetoxyisehutyrate~ c 44x 45.4 35.8b 7 4 8 146.7 51.6 80.6 47.2 74.1 34.gbI40.7 137.0 170.0 122.0 118.2 115.9 Crepiside E, pentacetate 1-31ti38014 c 320 4 5 9 372 80.3 147.4 52.1 77.8 47.7 73.9 37.8 141.6 137.5 169.0 122.5 118.3 1163 (‘IS111804 Cynaropicrin, 8-epi, desacyl 1’ 590 44.1 39 0 731 155.2 50.5 78.8 50.1 66.0 44.2 145.1 137.7 170.1 120.9 115.9 109.0 Cynaropicrin, X-epi, desacyl, glycnside C2IH2X09 I’ 590 45.2 38.5 80.8 150.8 50.0b 75.5‘ 50.7b 66.1 43.7 145.0 137.7 170.2 121.0 116.1 I l l y 45.1 38.5 80.8 150.8 50.0” 78.2’ 50.7b 66.0 43.7 144.9 137.7 170.1 121.0 116.1 1119 P 450 45.1 38.5 R0.7 150.8 50.7 78.5 50.0 66.0 43.7 144.9 137.6 170.1 121.0 116.1 111.9 I’ 472 Crepiside I C29H.34011 I’ 449 45.2 38.6 R1.l 151.0 50.7 78.1 49.9 66.0 44.0 145.0 137.7 170.1 121.0 116.2 112.0 Ixerin M C26H360l I 4 4 9 38.4 80.7 150.3 50.4 78.9 48.1 68.4 40.3 143.7 136.2 169.1 121.3 117.2 112.3 I’ 472 Ixerin M, pentacetate CXH46016 45.2 37.9 81.2 150.3 51.3 78.5 48.3 69.4 40.3 143.7 135.6 170.5’ 122.3 117.9 112 8 P 472 Ixerin N ( ‘ 2 7113x01I I’ 472 44.8 38.4 80.7 150.4 50.3 78.8 48.1 68.4 40.3 143.7 136.2 169.1 121.3 1172 112.2 C37H48016 lxerin N, pentaacetate I’ 412 45.1 37.9 81.3 150.5 51 2 78.6 48.3 69.5 40.6 143.1 135.6 169.2 122.3 118.0 112.6 Ixerin 0 C34H4201.3 P 477 44.9 38.3 80.8 150.3 50.5 78.9 48.2 68.4 40.3 143.7 136.2 169.3 121.4 117.3 112.5 C34H42013 Ixerin P P 412 4 5 0 38.6 81.2 150.2 50.5 79.0 48.2 68.5 40.7 144.0 136.3 169.3 121.4 117.3 1123 Ixerin Q C35114401.3 P 412 44.8 38.2 80.7 1.50.2 50.4 78.8 48.1 68.5 40.2 143.6 136.2 169.1 121.3 117.2 112.4 Ixerin R C3SH44013 P 472 44.8 38.4 81.1 150.5 50.4 79.0 48.1 68.5 40.7 143.8 136.2 169.3 121.4 117.2 112.2 C29H.36012 Tectrnside 4 4 7 3 8 4 80.7 150.6 50.3 78.9 48.1 68.4 40.7 143.7 136.1 169.4 121.6 117.2 1122 I’ 665 C2Yt/14011 Ixerisnside A P 665 44.4 38.2 80.7 150.6 50.1 78.8 48.3 68.2 40.6’143.5 136.1 169.4 121.4 117.1 111.9 C2IH2808 Ixerisnside D P 665 40.7b 33.2 30.0 153.1 51.7 86.1 39.3’ 37.1 82.3 151.6 140.5 170.3 119.6 112.4 108.5 CISH1702Cl laH,5aH-Guaia~(lS),1~~4),1~(13)-trien-lZ,~-olide, 90-chloro C 151 45.4 32.9 30.7 150.5 51.4 85.1 43.2 41.9 62.5 148.5 137.6 169.3 120.8 109.3 113.1 laH,5aH-Guaia-4(15),~0(14),11(13)-trien-~Z,~-olide,9p-hydroxy CISHI803 P 8 43.9 33.4 30.7 152.7 52.2 86.1 41.7 40.6 74.4 155.1 140.1 170.1 119.8 108.5 108.5 C2llI2808 Diaspannside B 43.X 33.4 30.7 152J 52.0 85.8 41.8 39.1 79.0 150.0 139.8 170.0 1200 111.1 108 7 I’ Y


371

Table 21.---continued Mol. formula Name / Chemical shifts C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-I2 C-13 C-14 C-15 Sol. Ref. 1109 C16H2003 laH,5aH-Gual~-4(15),1~14),11(13)-trien-12,~-ol1de, 2@-hydroxy-13-methyl 44.0 34.4 73.5 153.1 46.4 83.2 49.8 39.0 30.9 148.2 130.1 169.7 136.6 113.8 111.0 C

No.

>

Other carbons: 1083 iBur-3-CI-Z-OH: 171.7 74.7 51.1 23.4; 1084 iBuf-3-Cl-2-OTAC: 167.9 81.6 46.7 20 6: 1085a Glc: 102.2 75.3 78.3 71.9 77.9 62.9; 1085b Glc: 103.4 75.3 78.2’ 71.9 78.5’ 63.0; 1085c Glc: 103.3 75.2 78.5 71.8 78.1 62.9; 1086 Glc: 102.8 75.1 78.2 71.9 78.2 63.0 Phac-4’-OH: 171.3 41.1 124.8 130.9(2) 116.5(2) 158.2; 1087 Glc: 103.4 75.1 78.3 71.7 75.1 65.0 Phac4’-OH: 172.0 40.7 125.3 130.9(2) 11632) 157.9; I088 Ang: 166.8 127.2 139.9 15.8 20.4 Ac: 170.7 21.2 1089 iBut-2-OH-3-OAc: 170.4 73.6 68.7 22.3 169.9 20.9 Ac: 168.4 20.4; 1090 iBur-2.3-OAc: 169.4 77.2 64.9 19.4 169.2 20.0 168.8 20.3 Ac: 168.1 20.6 1091 Glc-2 6-Ac: 98.4 71.4 72.9 68.6 71.7 62.1 170.5 21.2 170.1 20.7 170.0 2 0 6 169.5 20.6 Ac: 169.1 20.6; 1093a Glc: 104.6 75.3’ 78.5‘ 71.9 78.2‘ 63.0; 1093b Glc: 104.6 75.3 78.2’ 71.X 78.5’ 63.0; 109% Glc: 104.5 75.1 78.3’ 71.8 78.0b 62.9; 1094 Glc: 104.5 75.0 78.5 71.6 75.0 649 Phac-4’-OH: 171.9 40.6 125.3 130.8(2) 116.2(2) 157.8; 1095 Glc: 104.1 75.2 78.2 71.9 78.2 63 0 iVal-2-OH: 174.1 76.1 32.5 17.2 19.2; 1096 Glc-2,3.4,6-Ac: 100.6 72.2’ 72.6‘ 68.9 72.2’ 62.5 a !L’u/-2-0.41, 170.0b 77.2 30.1 17.3 18.7 a ; 1097 Glc: 104.1 75.1 78Sb 71.9 78.1b 63.0 Val-2-OH-3-Me: 174.1 75 5 39.1 24.5 11.6 15.7; 1098 Glc-2.3.4.6-Ac: 100.8 72.3’ 73.6* 69.0 72.3b 62.6 a Val-2-OAc-3-Me: 170.2 76 X 36.6 24.9 11.5 15.4 a : 1099 Glc: 104.1 75Sb 75.7’ 73.0 76.0‘ 62.3 iVal-2-OH: 174 I 76.1b 32.5 17 2 19.2 Phac-4’-OH: 171.8 40.9 125.3 131.0(2) 116.2(2) 157.4; 1100 Glc: 104.6 75.2’ 78.3 71.8 76.1 65 I [Val-2-OH: 174.2 75.3’ 32.5 17.3 19.3 Phac-4’-OH: 172.0 40.7 125.4 131.0(2) 116.3(2) 158.0: 1101 G k 104.1 75.4b 75.9’ 72.9 75.6’ 62.1 Val-2-OH-J-Me: 174.0 75Sb 39.1 24.5 11.6 15.7 Phac-4’-OH: 171 7 10.X 125.2 130.9(2) 116.2(2) 157.8: 1102 Glc: 104.4 75.0b 78.2 71.7 76.0 65.0 Val-2-OH-3-Me: 174.1 75.5‘ 3‘1.1 24.6 11.6 15.7 Phac-4-OH: 172.0 40.7 125.3 130.8(2) 116.2(2) 157.8; 1103 Glc: 104.7 75.4 78 6 71 7 78.5 62.9 Cinn-diH-2.4-OH: 174.3 72.8 39.8 128.5 131.3 116.2 157.7 116.2 131.3; 1104 Glc. 104.7 75 4 78.7 71.8 78.5 62.9 Phac-4’-OH: 171.6 40.4b 124.9 131.1(2) 116.3(2) 158.2; 1105 G l o 103.5 75.4 7X 6 71.5 78.5 627: 1108 Glc: 101.9 75.5 78.6 71.9 78.6 63.0; 1109 C-16: 13.8

R’O O *R2

R’O

H i

0.

co-

R’

R’

1092

H

H

iBut-3-CI-Z-OTAC

1093

Glc

H

8-Glc

H

1094

Gk-6-OPhoc-4’-OH

H

8-Glc

Phoc-4’-OH

1095

Glc

iVal-2-GH

1087

P-Glc-6-OPhac-4-OH

H

1096

Glc-2.3.4.6-Ac

1Val-2-OAc

1088 1089

Ac Ac

Ang iBut-2-OH-3-OAc

1097 1098

Glc

Val-Z-OH-3-Me

Glc-2.3.4.6-Ac

Val-2-OAc-

1090

Ac

iBut-2.3-0Ac

1099

Gk-4-OPhoc-4’-OH

bVal-2-OH

1091

8-Glc-2.3.4.6-AC

AC

1100

Glc-6-OPhac-4’-OH

Wol-2-OH

1101

Gk-4-OPhac-4’-OH

Val-2-OH-3-Me

1102

Glc-6-0Phoc-4’-OH

Val-2-OH-3-Me

I103

Glc

Ccnn-diH-2.4’-OH

1104

Glc

Phoc-4’-OH

R’

R’

1083

H

iBut-3-CI-2-OH

1084

TAC

1085 1086

HO

0.

1105

co

R = a-OGlc

= 8-Cl

1106

R

1107

R = 8-OH

1108

R = 6-OGlc

co 1109

3-Me

372


Table 21.--continued No.

1110 1111 a b c 1112 a

b 1113 1114 1115 1116 1117

1118 1119 1120 a

h 1121

1122 1123 1124 1125 1126 1127 1128

I129 1130 1131 1132 1133 1134

1135

Mol. formula Name I Chemical shifts C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-I1 C-12 C-13 C-14 C-15 Sol I k l CISHI804 Znluzanin C, 9a-hydroxy 41.3 39.0 73.1 155.3 49.3 85.0- 35.9 40.0 72.7 153.5 141.0 170.2 119.1 112.0 108.0 P 473 C2IH2809 Macroeliniside A 41.6 37.5 80.8 150.9 49.5 84.4 37.0 39.8 72.3 153.3 141.2 170.1 118.9 111.1 112.2 P 447.45 I 41.6 37.5 80.9 151.0 49.5 84.6 36.8 40.0 72.5 153.4 141.2 170.3 119.2 111.0 112.3 P 327 41.6 37.4 80.7 150.9 49.5 84.4 37.0 39.8 72.3 153.2 141.1 170.0 118.9 111.2 112.2 P 472 Znluzanin C, 9f3-hydroxy CISHI804 42.1 39.6 72.7155.2 49.6 84.8 40.9 40.7 74.6155.0140.1 170.1 119.7110.4107.3 P X 40.7 39.1 73.2 152.6 49.5 84.3 42.1 40.7 74.5 153.0 138.3 169.8 120.8 110.9 109.9 c 350 C21H2809 Dimpanoside A 42.9 38.1 80.7 151.1 49.6 84.4 41.4 40.7 74.6 154.3 140.3 169.5 119.1 110.5 1107 I’ x CI 7H2006 Salograviolide A 48.7b 36.0 74.5 147.3 47.1b 79.3 40.9 77.9 79.8 136.2 147.8 171.1 125.7 113.2’112.8’ C 157 C19H2206 Repdiolide 46.5 77.3b 47.9 73.8 35.8 a a 169.1 135.9’ 137.2’ 139.4‘ 51.4 78.8b 78.9b a C 612* CI 7H2005 Rupicolin B, 8-acetyl-1-desoxy 47.7 36.6 126.3 140.9 56.7 80.2 49.3 74.3 38.4 142.6 137.0 169.6 122.0 116.8 17.2 c 331 C20H2406 Eupahakonenin B 47.6 37.3 126.6 139.6 56.4 80.0 48.5 68.2 40.3 143.1 134.4 169.4 122.0 116.4 I 6 7 c 333 C15H1603 Ligustrin, 8-deoxy -2-ox0 56.2 206.2 132.6 177.5 53.2 83.3 46.1 31.2 36.4 144.1 138.5 169.3 121.0 117.3 l9.X c 162 C21H2809 Crepiside D 44.7b 34.8 153.4 112.9 54.2b 81.7 51.4b 72.8 45.0b 144.7 139.7 170.1 121.1 115.7 12 I I’ 449 C2IH2809 Crepiside C = Ixerin T, desacyl 45.7 35.6 151.2 112.3 54.1 80.3 50.0 65.9 43.2 144.9 137.8 170.2 120.7 115.8 11 6 P 419 43.2 35.7 151.3 112.4 54.2 80.4 50.1 65.9 45.7 144.9 137.8 170.3 120.8 115.8 11.7 I’ 472 C29H34011 Crepiside F 45.2 34.7 151.4 112.9 54.3 E!.2 47.5 74.8 41.2 143.3 138.2 169.2 121.8 117.3 12.2 I’ 44IJ C26H36011 Ixerin S 42.4 35.4 151.3 111.4 53.9 80.7 48.3 68.2 42.2 143.6 136.0 169.1 121.1 116.8 1 1 5 1’ 472 C27H38011 Ixerin T 42.5 35.5 151.3 111.5 53.9 80.7 48.3 6R.3 42.5 143.7 136.0 169.1 121 I 116X I I i I’ 472 C20H2407 Eupahakonin B 84.5 47.8 125.4 147.0 64.8 79.7 47.8 68.7 36.7 139.2 136.3 169.1 121.1 115.8 1 7 1 A 333 CZOH2408 Eupahakonin B, peroxy I’ 33-3 95.4 43.3 125.6 144.5 59.8 79.3 48.5 68.0 37.1 137.5 135.5 169.3 121.5 1 1 8 1 169 C20H2406 Preeupatundin, 8~-(4’-hydroxytigloyloxy) 52.5 78.4b 129.3 147.T 56.0 81.0b 47.8 68.2 38.6 141.6‘ 134.2’ 169.7 122.3‘ 119.3‘ 17.2 c‘ 17 C22H2608 Preeupatundin, 8p-(4’-hydroxy-S’-acetoxytigloyloxy) I. 631 53.8 79.1 129.9 147.3 56.4 81.3 48.3 69.0 39.3 142.1 134.9 169.6 122.2 119.4 174 C22H2607 Eupachifolin C c 112 50.7 80.3 126.3 148.2 56.0 80.0 48.0 68.0 39.0 139.1 134.0 169.2 122.4 120.1 17 Z C22H2608 Eupahakonesin (‘ 333 50.6 80.2 126.1 147.9 55.8 80.0 47.9 68.3 39.0 138.7 133.7 169.1 122.5 120 1 17 2 C25H3008 Pericomin c 307 46.9 37.4 126.9 143.Ib 48.6 80.3 56.1 67.7 41.2 134.4b139.4b170.3 122.8 116.4 592’ C25H3008 Pericomin, is0 c 367 47.2 37.3 126.8 143.1b 48.4 80.0 56.2 68.2 40.6 134.5*139.6b169.9 122.0 116.4 59.1‘ C20H2405 Achifolidien, is0 c 540 146.1b132.2 127.4 149.c 58.6 83.4 42.2 31.8 76.0 75.3 139.5 169.9 119.2 27.4 17.0 C19H2406 Cyclotagitinin C c I40 50.4 49.0 207.6 134.2 160.9 75.7 46.9 65.2 37.3 73.4 141.3 168.1 122.0 23.5 9.5 C19H2306CI Guaia4,11(13)-dien-l2,6a-nlide, la-chloro-10a-hydroxy-8P-isohutanoyloxy-3-1ixii 75.8b 47.0 203.4 132.9 162.8 75.0 46.9 65.4 39.1 75.9b 137.9 168.6 123.2 29.5 X.X C I40 C19HZZ06 Guaia4,11(13)-dien-12,6a-olide, la,l0a-epoxy-~-oxo-8~-isnbu~dnl~yloxy 63.2b 39.3 202.4 132.9 155.0 74.3 42.6 64.4 34.8 68Sb146.6 168.1 122.4 23.7 99 c‘ I40

373

CARBON-I3 NMR SPECTRA OF SESQUITERPENE LACTONES

Table 21.4ontinued ~~

-~~

~

Mol. formula Name / Chemical shifts C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-11 C-12 C-13 C-14 C-15 1136 C19H2206 Gunia4,11(13)-dien-l2,~-oiide, 1~,10~-epoxy-3-oxo-8~-isobutanoyloxy 64.9b 40.4 202.7 133.8 159.0 75.3 48.2 64.5 37.4 67.6b 141.4 167.9 122.5 25.7 9.0

No.

Sol

ICC!~

C

I?O

Other carbons: l l l l a Glc: 104.4 75.3 78.2 72.0 78.6 63.1; l l l l b Glc: 104.7 75.5 7X.7 72.0 7X.5 63.1 l l l l c Glc: 104.2 75.2 78.5 71.8 78.0 63.0 1113 Glc: 104.6 75.3 78.5b 72.2 77.9h 63.3; 1114 A c - 1701 21.0; 1115 Mac: 166.5 a 147.2' a ; 1116 Ac: 168.9 21.3; 1117 Tig-4,5-0H: 165.8 131.3 143.9 5 X X 56.9; 1119 Glc: 102.0 74.9 78.6 71.5 78.5 62.6; 112Oa Glc: 101.8 74.9 78.4 71.4 78.4 62.5. 112Oh (;/i 101.8 74.9 78.4 71.4 78.4 62.5; 1121 Glc: 102.1 75.0 78.7 71.6 78.6 62.6 P/iac-4'-OH: 171.2 41.2 125 (> 131.0(2) 116.5(2) 158.3; 1122 Glc: 101.6 74.8 78.3 71.4 78.3 62.4 iVal-2-OH: 174.0 75.9 72 3 17 (1 19.3: 1123 Glc: 101.7 74.8 78.3 71.4 78.3 62.4 Val-,?-OH- -3-Me: 174.1 75.5 39.0 24.3 I 1 5 15 X . 112-1 Tig-4.5-OH: 166.2 132.2 145.9 59.1 56.9; 1125 Tig-4.5-OH: 166.4 132.1 145.8 58.7 56 3; 1126 7 1 , y - 4 - ( J / / 166.8 127.5 141.6 59.3 12.5; 1127 Tig-4-OH-5-OAc: 165.5 127.4 147.6 59.6 58.2 171.4 21 0: 1128 Tig-4-OH: 166.5 127.8 141.1 59.6 12.7 Ac: a 21.3; 1129 Tig-4,S-OH: 165.8 131.1 144 0 5 X X 50 X A S 170.1 21.3; 1130 Ttg-4-OH: 167.4 127.2 142.0 57.9' 12.4 Sac 165.1 126.6 I48.Ob 59." 16.5: 1131 S i r , 165.0 126.5 147.9b 5 8 2 16.5 Mebu-2.5-en-4-OH: 166.9 136.6 35.0 61.1 128.0. 1132 Tig. 166.X 1% 3 I X 2 14.6 12.3: 1133 iBur: 175.7 34.2 19.1 18.9; 1134 iBut: 176.6 34.1 19.2 18.9; 1135 rllrrr. 175 0 31(1 I 0 I 18.9: 1136 iBut: 176.2 34.1 18.9 18.9 HO

* '4'

H (

1110

R = H

1112

R = H

1111

R = Gk

1113

R = Glc

1114

0.

co

v.

c0 '

1115

GlcO

1116

R = a-OAc

1117

R = 6-OTig-4.5-OH

R'

R'

1126

H

Tig-4-OH

1127

H

Tig-4-OH-5-OAc

1128

Ac

Tig-4-OH

1118

R = H

1130 R = Tig-4-OH

1119

R = a-OH

1124

R = H

1120

R = 6-OH

1125

R = OH

1121

R = a-OPhac-4'-OH

1122

R

1123

R = 6-OVol-2-OH-3-Me

1133

R = H R = Cl

-

1132

1131 R = Mebu-2.5-en-4-OH

1135

fl-OiVoi-2-OH

1134

1136

OMac

374

M. BUDESINSKY AND D.

SAMAN

Table 21.-continued Name / Chemical shifts

Mol. formula

No.

C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-I1 C-12 C-13 C-14 C-IS Sul-I(cl: 1137 ClYH2306CI laH-Guaia-4,11(13)-dien-12,6a-oliie, 2-chlor~l0a-hydroxy-E~-isobutanoyluxy-3-11~11 5 5 3 61.5 200.4 133.6 153.8 75.1 46.8 64.8 49.1 73.6 140.2 167.7 122.7 23.8 10.1 Zaluzanin C, 3-desnxy-4a,l5-dihydro-7a-hydroxy 1138 CISH2003 44.6 27.8 35.1 36.1 43.3 86.0 75.5 36.9 34.3 143.5 150.1 170.0 122.6 1 1 1 5 15.1 Estafiatone ll3Y CIS111803 39.9 43.9 218.8 47.0 50.8 88.7 44.1 31.8 38.5 148.7 138.9 169.7 121.0 113.0 14.3 1140 CISHI804 Grossheimin d 39.6 42.9 218.9 46.6 50.5 82.5 48.8 72.4 48.0 143.7 136.4 169.8 125.1 114.5 I4 4 h 404* 43.5’218.7 47.2 49.8’ 83.3 51.1 73.2 49.2‘145.4 138.7 170.3 124.5 114.4 15.0 c 45.9 47.1 218.1 39.3 48.5 82.0 49.9 71.7 42.4 144.4 137.7 169.6 123.2 113.8 14 2 C-l

C-2

C-3

1141 CliH200.5

43.2’ 46.8’218.2 1 I42 ClYH2407

Grossheimin, 8-0-acetyl 40.6 46.4b 82.2 51.1 74.3 43.0’142.4 136.0 171.1 124.6 116.7 Grossheirnin, 2’,3’-dihydroxyisobutyrate

40.7 42.3b218.2 46.7’ 51.5 80.6 46.5’ 75.9 40.7’14l.5 135.6 174.7 125.2 117.3 1143 C23H2809 Grossheimin, 2’,3’-diacetoxyisobutyrate 4 1 . 1 42.4b217 7 46.7’ 52.0 80.6 47.2‘ 75.8 40.0* 141.6 136.4 170.0 124.3 117 7 1144 CZOH2606

14.9 15 1 15 4

Ixerin S, aglycone

47.2 50.7 81.8 47.2 69.0 43 8 143 4 134.5 1686 122 7 1164 1-1 1 Zaluzanin C 3-dehydro-4~,15-dihydro-8~-(4’-hydroxy-5~-(4’~-hydroxytiglii~ Iiixy)tigIo>I i i x ? 79 8 43 9b219.5 47.1 50.1 82.7 47.1 67.9 44 3* 143.6 134 5 1699 123 4 116.3 I4 2 40.2 44.2b219.1 47.5 50.7 83.2 47.5 69.2 44.5b 145.6 136.6 1697 122 I I15 9 14 4 C17H2204 laH,SaH-Guaia-10(14),11(13)-dien-12,6a-olide, lS-ethoxy-3-oxo 4 0 0 45.7 217.7 44.6 44.2 88.8 52.9 38.4 31.8 139.0 148.9 169.8 121.2 113.0 681 0 CI5H1803 Estafiatin 44.8 32.9 62.2 65.0 5 0 7 79.4 43.8 28.8 2 8 . 5 1 4 6 . 6 1 4 0 . 4 I h X 3 l l X . 4 1 1 4 3 1 x 6 C20H2406 Eupatundin, 2-desoxy 58.1 74.3 6 4 3 67.1 80.3 78.5 41 9 6 8 7 37.7 139.2 174 5 16‘1 I 122 4 121 7 I5 X C20H2207 Eupatundin, dehydru 61.2 204.2 62 I 6 8 4 77.7 77.8 41.4 68.1 3 5 9 133 2 1346 1689 l 2 4 X 1 2 3 X I T 7 C‘ZjH3208 Zaluzanin C, 4~,1~-dihydro-R~-(5’-(5”-hydruxyti~loyluxy)-tiglii)loxy) 42 6 38.5 78.3 46.4 51.9 80.8 50.6 66.9 40.9 1423 1350 I69 2 121 5 I l h X 176 400

44.5 218.1

1145 C25H.3009

a h

1 I46

1147 I I4X 1 l4Y

1150

51 I 35.6 1152 C30HWOlZ

43.3‘ 38.5 1153 CI 5H2004

79.8 43.5

43.0

80.6 50.4 67.2 41.6 142 I 134.8 1693 122.0 117 I

Ixerisnside B

87.4 44.gb 51.4 81.6 49.9 67.8 41.6 143.5 136 3 169 3 121 2 116 X Grmheimin, 3-dihydro

51.0 81.6 53.6 72.8’ 46.6b 144.4 139.8 171.4 121.8 114.2 Grossheimin, 3-dihydro-3,S-di-O-acetyl 43.7b 49.8’ 80.0’ 35.6 43.5’ 80.5 5 1 6 74.4* 40.4‘ 141.5 137 4 169 7 I22 4 I17 I 1155 C19H2406 Subcordatolide A 85.7’ 140.2 134.6‘ 82.0’ 66.5‘ 67.3* 46.2 76.8 35.8 135.0 144.0 176.0 122 I I I h X I156 C20H2406 Subcordatolide A, 8-desaryl-8-tigloyl X5.8* 140.2’ 138.2’ 82.1b 66.Sd 67.6* 46.4 76.9d 36.0 134.8 144.1 169.8 122 6 1169 laH,5aH-Guaia-10(14),11(13)-dien-12,6a-olide, 3p,4a-dihydroxy 1157 C15H2004 38.8 34.3 78.3 80.5 53.9 83.7 47.4 3 1 6 39.1 148.9 140.4 170.1 119.4 112.5 Diaspannside C ll5X CZI H300Y 38.7 32.9 85.1 80.7 53.3 83.4 47.5 31.7 39.2 148.3 140.3 170.2 119.6 112.6 Michefuscalide = I,ipiferolide, p-cyclo 1159 Cl711220S a 44 0 26.1 44.8 79.9 55.4 78.7 49.2 74.3 39.8 141 I 135 5 I6X 5 127 7 115 7 h 44.2 26.4 404 79.X 5 6 4 77.9 5 0 2 6 5 6 43.7 141.i 134.8 Ih9O 1220 1167 Cebelin C 1160 C19112307CI 48.9 35.8 78.5 85.8 59.6 77.0 49 3 75 5 40.0 I41 9 I449 170‘1 122 7 I 1 7 6 Cebelin C, 3v,4’-diacetyl 1161 ( X H 2 7 0 Y C [ 46.7 35.2 78.4 83 8 57.9 75.6 46 8 74 I 36.2 140.9 1364 16X 4 123 I I I X 9 Chlororepdiolide 1162 C23H270iCI __ 5 8 4 83.3 84.1 83.9 60.1 77.6 47.2 74.4 3 6 . 7 1 4 2 . 8 1 3 8 . 8 1 6 9 . 0 1 2 0 . ‘ ~ 1 1 8 0 46.8

39.0h 77.5‘ 42.9

1154 ClYH2406

375


Table 21.--continued Othercarbns: 1137 iBur: 175.6 34.2 19.1 18.9;1141 Ac: 169.4 21.0; 1142 iBur-2.3-OH: 168.9 75.5 67.9 21.8;1143 iBur-2.3-OAc: 168.7 78.6 65.4 20.0 169.3 20.6 169.8 20.8; 1144 iVnl-2-OH: 174.2 74.9 32.1 15.9 19.2; 1145s Tig-4-0H-S-OTig-4'-OH: 165.2 126.7 148.9 59.5 58.1 167.4 127.3 142.3 59.2 12.5: 1145b Tig-4-0H-li-OTig-4'-OH: 165.7 127.4 150.1 59.6 58.7 167.6 127.4 143.6 59.3 12.6;1146 OEI: 66.7 14.9;1148 Ang: 167.3 127.4 138.7 15.8 20.5; 1149 Ang: 166.9 127.1 139.1 15.8 20.5; 1150 Tig-S-oT~g-5'-OH: 165.4 127.6 145.7 14.4 56.4 166.8 131.6 141.4 14.0 57.5: 1151 Tig-4-OH-5-OTig-5'-OH: 165.2 126.8 147.9 59.2 56.4 167.0 131.6 142.1 14.3 58.1 Ac: 171.2 21.2;1152 Cinn-diH-2.4-OH: 174.4 72.8 40.9 128.7 131.2(2) 116.2(2) 157.7 Glc: 105.8 75.5 78.6 71.8 78.4 62.9;1153 2xAc: 170.9 21.1 169.9 22.0;1155 iBut: 169.7 34.0 19.0 18.8;1156 Tig: 166.9 138.8 134.8 14.4' 12.W;1158 Glc: 104.6 75.9 78.7h 71.8 78.5' 62.9;ll59a Ac: 169.3 21.0 1159b Ac: 170.3 20.9;1160 Mnc-4-OH: 166.6 139.3 126.0 61.6: 1161 Mnc-4-OAc: 164.3 135.0 129.4 62.2 169.2 21.2 Ac: 170.3 20.8;1162 Mnc: 166.5 136.7 126.3 IK.3 0

*

OiBut

0.

co

1137

1138

-

1139

R

1140

R = OH

1141

R = OAc

1142

R

1143

R = OiBut-2.3-OAc

=

H

1144

R = iVol-2-OH

1145

R = Tig-4-OH-5-OT!g-4'-OH

OiBut-2,J-OH

R'

Rz

R'

R2

1147

H

H

1150

H

lig-5-OTig-5'-OH

1148

OH

OAng

1151

Ac

Tig-4-OH-5-OTig-S-OH

1152

Glc

Cinn-diH-2.4

1146

R'

Rz

1155

R

1153

OH

H

1156

R = lig

1154

OAc Ac

=

1149

-OH

R'

R2

R'

R2

R'

1157

OH

H

1160

H

H

Mac-4-OH

1158

OGlc H

1161

H

AC

Mac-4-0Ac

1159

H

1162

OH

H

Mac

iBut

OAc

376


Table 21.4ontinued Nu

1163 1164

I165

1166 1167 a h 1168

1169 1170 1171

1172

1173 1 I74 1 175

I 176

1177 a h c

Mol formula Name I Chemical shifts C-I C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-LO C - l l C-I2 C-13 C-14 C25H2Y010CI Cebelin C, 38,4,4‘-triacetyl 46.6 34.8 76.7 92.8 54.5 75.3 47.0 74.0 36.0 140.4 136.3 168.4 123.1 119.3 C19H2207 Sublutenlide 45.4 37.6 75.1 68.2 97.7 75.9 52.8 75.6 36.5 141.4 136.6 168.9 118.4 123.2 ClYH2207 Repin 46.0 38.9 75.5 69.2 53.1 77.5 47.8 75.3 36.4 142.7 138.6 169.0 121.1 118.0 C20H2407 Sublutenlide, 8-desacyl-8a-(4‘-hydroxytigloyl) 48.1 47.6 74.1 68.2 45.6 76.9 53.0 76.1 36.6 141.4 137.1 169.1 122.7 118.5 ClYH2207 Janerin 46.3 36.8 76.0 69.0 53.5 77.6 48.2 74.7 38.9143.5139.1 169.3121.5118.1 47.Yb 37.6‘ 74.2’ 68.2 45.7b 76.7‘ 53.1 76.3‘ 36.5’ 141.4 137.0 168.9 122.7 118.6 C191f2207CI Acroptilin 46.7‘ 36.1‘ 76.2* 69.0 54.0 76.3 48.Ib 76.2‘ 39.2’ 143.3 139.0 169.3 121.9 118.5 C19H2307C1 Costuslactone, dehydro, 2a,3~dibydroxy-4a-epoxy.&r-tigloyloxy 49.2’ 73.1 77.2 65.4 47.1b 77.4‘ 51.7 78.9’ 36.7 135.2 136.9 168.4 121.9 119.4 CI5HIBO2 Eremanthin, is0 47.9 38.1 125.5 143.3 54.9 86.1 45.0 30.0 119.5 137.3 139.3 170.0 119.3 17.8 C17112004 Cumambrin A, 9,lO-dehydro 47.5 37.9 125.6 138.2 55.1 79.5 46.8 73.1 121.6 142.7 136.6 169.7 122.9 27.7 CISHIR04 Rupicolin A 83.3 46.3 123.1 141.9 64.2 78.2 50.0 71.0 127.6 139.0 137.7 169.8 123.6 29.4 CZOff2407 Eupahakonin A 83 4 47.7 123.6 145.5 66.4 79.0 48.2 67.8 119.6 142.2 135.9 169.0 121.2 24.9 C20H2408 Eupahakonin A, peroxy 94.6 44.2 123.4 142.7 60.7 78.9 48.2 67.9 122.8 141.7 135.6 169.0 121.6 24.4 C25H3009 Rupicolin A, 8-epi, 8-(5’-(4”-hydroxytigloylnxy)-tigloyloxy) X3 3 47.0’ 119.2 141.7 47.6b 65.5‘ 66.9’ 76.8 123.0 144.5 134.2 169.7 122.6 24 3 C19112206 Helipterolide, 14-acetate-3f3-acetoxy I41.0 37.9 82.0 146.2 53.1 76.4 51.6 2 6 0 30.5 140.8 132.0 168.7 1178 66.7 ClS111802 Eremanthin 47.1 29.2 30.5 150.4 52.7 83.2 45.3 29.7 121.2 138.2 140.7 170.1 119.4 27.2 47.1 29.2 30.6 150.3 52.7 83.2 45.3 29.7 121.1 138.0 140.4 169.9 119.3 27.9 47.0 30.5 29.1 149.9 52.5 83.1 45.2 29.6 120.8 137.8 140.1 169.7 119.1 27.9

1178 C24113009 ~ 4 n4 1 2 1179 C2611iZOlO

C-15 42.7

C

48.3

C 453*

48.6

P

48.4 48.5

c+s

51.4 47.3

E

424 34

E

424

c+s

34

28 0

c

IS1

17.6

c

333

17 6

c

705

17 9

A

333

I7 8

A

333

17.5

c

I99

116.3

13

699

110‘9 1109

c c

229

1107

c

305

Eremanthin, 3~-hydroxy-8a-(2’,3’-diacetoxy-2’-methylbutanoylnxy~ 72.1 152.2 51.1 77.2 48.8 67.2 119.9 145.2 134.6 168.9 122.1 28.4 115 2 Eremanthin, 3~-acetoxy-8a-(2’,3’-diacetoxy-2’-methylbutanoyloxy~

c

343

c

343

c

343

c

343

P

633

c 40.3 145.4 137.8 170.6 122.8 115.7 17.2 la,lO~-dihydroxy-3~-is0butanoyloxy-3-nxo c 40.3 81.3 138.4 168.2 123.6 22.5 86

705

C

162

c

402

44.0 38.6 11x1 C24112809

73.5 147.3 51.5 76.9 49.2 65.6 120.6 144.1 135.5 168.9 121.3 28 3 117.7

809 46.2 11x2 (‘20112207

72.6 147.0 60.9 76.6 47.6 65.5 120.9 144.6 134.9 168.9 121.9 2 4 8 118.2

Eremanthin, Eremanthin,

151

3P-acetoxy-8a-(4’-acetoxyangeloyloxy)

3~-acetnxy-8a-(5’-acetoxyangeloyloxy)-la-hydrnxy

Montacephalin

135.6 193.3 134.8 173.1 78.4 84.4 48.5 70.1 11x3 CIS111804 Rupicolin B 8 4 0 45.2 124.5 140.0 64.6 79.8 50.9 72.2 1 1x4 CIYII2407 Guai-4,11(13)-dien-12,6a-olide, 74 X 46 3 204.1 132.6 161.9 73.9 48.1 63.9

~~

613

C+S 34

73.2 147.3 51.3 76.4 48.6 67 I 120.1 144.6 134.4 168.7 122.1 28.2 117.6

132 0 195.6 I 1x6 c1s1116i04 a 134.2 195.5 h 132 R IO6.O 11x7 (‘20112206 a 174 2 I04 7 h 132.hb194.9

126

48.5

44 0 38.4 I I X O (’241126‘08

11x5 (‘1511160.~

Sol. Ref

74.8 148.0 139.1 169.3 121.9 13.1

156

140

Leucndin, dehydro

135 6 169.4b 53.0 84.4 53.1 24.5 37.2 151 7 138.6 169.0b118.6 21.8 I9 7 Matricarin, 1l,l3-dehydro, desacetyl = Lactucin, 14-deoxy 1360 170.7 52.0 82.5 5 8 3 6d.2 49.4 146.2 139.2 169.5 121 8 21 3b 198’

A 705 135.2 170.6 51.4 81.6 57.5 67.0 48.5 146.7 136 7 169.6 122.7 19.5 21 3 5aH-C.uai-1(lO),3,11(13)-trien-l2,(nr-~lide, X~-(2’,3’-epoxyangeloyl1~xy)~2~11~1~ c‘ 21‘9 135.5 168 7 54.5 78 4 52.6 65.4 40.6 145.4 132.4 169.0 120.0 22.8 18 7 135.9 169.1‘ 53.0 78.6 55.1 65.6 41.1 145.8 134.3b 167.8 120.4 23.2d 19.7’’ C 262

377


Table 21.-continued Mol. formula Name / Chemical shifts C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-12 C-13 C-14 C-15 Sol I r O

co

6

J

Epmg

0-co

2000

1999

2004

2005

2002. 2003

2001

2006

R = a-OEt

2007

R = 8-OEt

2008

CrilOAnq

Hoow 2010

2009

2014

2013

7

2012

2011

2015

R = H

2016

R = OH

0

2022

R = H

2025

R = Ez

2023

R = GI

2026

R = Ez-4'-OH

R'

R2

2017

Oh

OH

2018

Obc

OAc

2019

OEz

OH2

2020

on

CI

2021

OA

C1

450


Table 31.-continued ~-

~

___

h l d lomula N.we / Chemical shifts C-I C-2 C-4 C-4 C-5 C-6 C-7 C-8 C-9 C-I0 C - l l C-12 C-13 C-14 C-15 2027 CISH2203 Cinnamolide, ’)a-hydraxy 41.6 17.9 31.3 32.7 41.7 25.3 141.1 130.0 77.3 38.5 74.3 169.3 17.0 21.5 33.3 202X C17112h03 Cinnamolide, 11-ethoxy 393 18.5 42.3 33.1 49.7 25.1 136.0 128.2 57.6 34.1 104.3 1678 21.4 33.2 1 5 3 202Y CISH2402 Confertifolin, dihydro 40.4 18.1 42.0 32.9 51.4 18.4 22.4 37.4 49.9 35.4 67.6 179.1 14 5 22.0 31 5 2030 C241f3207 Pebrolide 82.4 27.4 35.1 36.8 46.4 67.6 28.0 35.9 48.1 39.3 70.1 178.4 117 IY.6 7 2 0

Nu

Sol. I

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