Barry J. Say

Perspectives in high-resolution solid-state nuclear magnetic resonance, with emphasis on combined rotation and multiple-pulse spectroscopy

Recent trends of general interest in high-resolution NMR of solids are mentioned, and some examples briefly discussed. Stress is laid on the use of the multiple-pulse technique, particularly when combined with magic-angle spinning to give CRAMPS. Two studies of proton CRAMPS are described, one concerning hydrogen bonding in carboxylic acids and the other involving phosphonic acid derivatives. The relationship between proton isotropic chemical shifts and hydrogen-bond distances is explored in some detail. A clear correlation is found.

High resolution 13C n.m.r. spectra of solid isotactic polypropylene

Abstract The high-resolution 13 C n.m.r. spectra of three samples of solid isotactic polypropylene are reported. The spectra, obtained under conditions of proton dipolar-decoupling and fast magic-angle rotation and using cross-polarization, are of annealed and quenched samples of the α-crystalline form and of a sample of the β-crystalline form. Attention is drawn to the importance of knowing the proton relaxation characteristics in these experiments and some illustrative proton T 1 ϱ data are given. The 13 C n.m.r. spectrum of the annealed sample of the α-crystalline form shows well-resolved splittings of the methyl and methylene resonances in a 2:1 intensity ratio. These splittings are interpreted in terms of the crystal structure of the α-form as suggested by X-ray diffraction. Quenching the α-form causes significant changes in the spectrum including a loss of resolution of the splittings obtained from the annealed sample. The β-form shows broad symmetrical resonances for the methyl and methylene carbons. The chemical shifts and other spectral features are discussed in the light of the proposed crystal structures and the effects likely to be produced by quenching.

Chemical Applications of High-Resolution

13C n.m.r. experiments are described that use the dipolar decoupling, cross-polarization and magic angle rotation techniques with a home-built spectrometer system. The subsidiary experimental methods of (a) spin-echo Fourier transform, (b) magnetization flip-back, (c) non-quaternary suppression and (d) single-pulse excitation are mentioned, and the equipment is briefly described. Applications of the methods are presented that have involved organic compounds, polymeric materials and inorganic systems. Discussion centres on special features of solid state spectra such as (a) multiplicity of crystallographic sites, (b) chemical shift and bandwidth differences from solution state spectra, and (c) effects of coupling to heteronuclei. Emphasis is placed on the value of the experiments for structural chemistry, as in cases of tautomerism, polymorphism and hindered intramolecular motion.

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Abstract Solution-state 13 C nuclear magnetic resonance spectra are presented for the emeraldine base and leucoemeraldine base forms of polyaniline. The bulk of the leucoemeraldine base sample gives a simple two-line spectrum, which is readily assignable. The emeraldine base samples give spectra of much greater complexity, showing many more lines than there are carbons in the postulated chain repeat unit. This is rationalized in terms of slow interchange of the many conformers available to the emeraldine base form. A quantitative ‘region assignment’ is made. Gel permeation chromatography measurements show that interconversion between the two forms is achieved without either significant crosslinking or chain scission. The minor peaks in the spectrum of the leucoemeraldine base form due to chain defects (including chain ends) are compared with chemical shift values calculated for postulated structures, and some structures can be rejected on the basis of this.

C n.m.r. Spectra for Solids [and Discussion]

SummaryCross-polarization spectra of solid polymers can be misleading when the material exhibits proton spinlattice relaxation in the rotating frame which cannot be represented by a single exponential. Quantitative and qualitative demonstrations of the problem are given. The use of two experimental procedures for alleviating the difficulties is suggested, and examples for polyalkenes and for polyethylene terephthalate are illustrated.

Solution-state carbon-13 nuclear magnetic resonance studies of polyaniline

Abstract Two samples of isotactic polypropylene (PP), with different treatment histories, were studied by carrying out a number of proton n.m.r. measurements, namely free induction decay (FID) and spin-lattice relaxation in both the laboratory and rotating frames. The results were subjected to detailed mathematical analysis, and are shown to yield information about the domain structure and intrinsic relaxation parameters. Inversion-nulled FID analysis shows that the samples contain a spatially resolved distribution of crystallinities, which is responsible for bi-exponential spin-lattice relaxation.

Discriminatory experiments in high-resolution13C NMR of solid polymers

Abstract 13C CPMAS spectra have been recorded for nine samples of cortisone acetate (21-acetoxy-17α-hydroxy-4-pregnene-3,11,20-trione, C23H30O6) and the observed resonances are attributed to six crystalline forms. Two of these forms are novel. The spectra of all six are entirely distinct. The resonances are assigned to individual carbon atoms on the basis of previously-determined solution-state spectra together with dipolar dephasing experiments and observations of shielding anisotropies as reflected in spinning sideband manifolds. The results are interpreted in relation to known crystal structures. The value of solid-state NMR for analysis of polymorph mixtures is emphasized.

Solid-state 1H n.m.r. studies of polypropylene

Abstract Numerical simulations of the NMR spin-lattice relaxation behavior in a number of models for solid polymers of lamellar morphology are presented. These give rise to an insight into the relationship between observed and intrinsic relaxation behavior for such a system in the presence of spin diffusion. The predictions of the model are used as a basis for discussion of experimental measurements on a sample of isotactic polypropylene.

Cross-polarization/magic-angle spinning NMR studies of polymorphism: Cortisone acetate

The use of high-power proton decoupling has enabled highly-resolved spectra of fluorine polymers to be recorded, as is exemplified herein for semicrystalline poly(vinylidene fluoride) (PVDF). By means of high MAS speeds (up to 17 kHz), the spinning sidebands are removed from the whole of the relevant chemical shift range. For spectra of the crystalline regions of the polymer, the high-power decoupling is necessary, though its effect is not large. Various relaxation techniques have been used to examine the semicrystallinity and the polymorphism of PVDF, with special pulse sequences used to discriminate between the various domains. Different chemical shifts have been observed for the signals of the amorphous and crystalline phases. Those of the more immobile parts cover a substantial range.

Numerical simulations of the effects of spin-diffusion on NMR spin-lattice relaxation in semicrystalline polymers

Abstract Carbon-13 CPMAS spectra have been recorded for anhydrous and monohydrate forms of androstanolone. The results are discussed in relation to crystallography. The anhydrous form gives crystallographic splittings in the spectra arising from non-equivalent molecules in the unit cell. The spectra are assigned by comparison with solution-state data.