Solid-state nuclear magnetic line narrowing

Solid state materials have been studied by nuclear magnetic resonance methods over 30 years. In 1953 Wilson and Pake ) carried out a line shape analysis of a partially crystalline polymer. They noted a spectrum consisting of superimposed broad and narrow lines which they ascribed to rigid crystalline and amorphous material respectively. More recently several books and large articles have reviewed the tremendous developments in this field, particularly including those of McBrierty and Douglas 2) and the Faraday Symposium (1978)3) —on which this introduction is largely based. 

The broad lines obtained for solid-state NMR spectra without applying any line-narrowing improvements are due to the different behaviour of nuclear spin interactions in solids compared to liquids. These interactions are averaged to zero or reduced to the isotropic values in liquids by the fast molecular motions, whereas the fixed (and different) orientations (with respect to the external magnetic field Bq) of the local environments of NMR active isotops in the rigid lattice of a solid cause line broadenings. The recorded broad NMR line patterns are superpositions of resonances from randomly oriented individual nuclei due to a random distribution of different orientations, since zeolitic materials usually are microcrystalline powders. Table 1 summarizes the nuclear spin interactions and their behaviour in liquids versus solids (17). 

Nuclear magnetic resonance (NMR) spectroscopy is at present one of the most widely applied physical techniques in biology, and its potential applications increase day by day, as more sophisticated instrumentation becomes available and deeper theoretical knowledge is obtained. The phenomenon of NMR was discovered simultaneously by Purcell and his associates at Harvard University and by Bloch and co-workers at Stanford University, for which they were jointly awarded the Nobel prize in physics in 1952. In the lipid field there are two main types of NMR spectroscopy that are of interest broad-line experiments, concerned mainly with the spectra obtained from samples in the solid state, or from oriented phases, and narrow-line, or high-resolution, experiments carried out with samples in the liquid, solution or gas phases. Both types of NMR spectroscopy are extremely useful in the study of the lipids. In addition, Fourier transform (FT) NMR has helped increase the sensitivity of the technique and the so-called pulse method of recording spectra has literally widened the prospect of NMR applications in the field of lipid research and industry. The application of NMR to solid fats is still in its infancy (Pines et aL, 1973 Schaefer and Stejskal, 1979 BocieketaL, 1985). 

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