High-resolution NMR in solids

Nuclear magnetic resonance spectroscopy operates at the low-energy end of the electromagnetic spectrum and allows small energy differences, corresponding to chemical shifts and coupling constants, to be measured. The 0.1-Hz resolution which can be routinely achieved corresponds to an energy difference of 10 Joule/mole. Such subtle differences could not be measured in solids in the past for three principal reasons. 

4 Chemical Shifts and Spin-Spin Couplings in C-NMR Spectroscopy 

Another factor which has posed problems in the past in recording the NMR spectra of solids has been the very long spin-lattice relaxation times in solids. Since the delays between successive pulses have to be 5T in order to allow the nuclei to relax fully to their original equilibrium state of magnetization, the long relaxation times in solids make the intervals between pulses in FT spectroscopy so great that only a few scans could be accumulated, resulting in a reduction in sensitivity. 

The three major problems in the recording of high-resolution NMR of solids mentioned above have been solved by (a) high-power decoupling, (b) magic angle spinning (MAS), and (c) cross-polarization (CP). 

The problem of long relaxation times of nuclei in solids has been overcome by utilizing cross-polarization (CP) and proton enhanced (PE) spectroscopy. By this process the C spin system transfers its energy to the spin system. By relaxation of the protons, the energy of the C nuclei can then be transferred to the lattice. Since the spin-lattice relaxation times are 

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M. Mehring. Principles of High Resolution NMR in Solids. Springer Verlag,... 

U. Haeberlen. High Resolution NMR in Solids. Academic Press, New York, 1976. 

Mehring M (1983) Principles of high resolution NMR in solids. Springer, New York... 

M. Mehring. Principles of High Resolution NMR in Solids, volume 11 of NMR-Basic Principles and Progress. Springer-Verlag, Berlin, second, revised and enlarged edition, 1983. 

It is interesting to note that several of the concepts for improving NMR technology, as listed by Levy and Craik, in 1988, already have been partially or fully achieved (1) two-dimensional Fourier transform (FT NMR) (2) high-resolution NMR in solids (3) new types of pulse sequences (4) chemically induced dynamic nuclear polarization (5) multiple quantum NMR and (6) NMR imaging (MRI). 

Haeberlen, U. High Resolution NMR in Solids Selective Averaging Academic Press New York 1976... 

The Encyclopedia of NMR is an excellent source, with more than 10% of the articles covering the broad area of solid state NMR. Included are a number of articles (too many to list here) that deal with high resolution NMR in solids and provide information on various sample spinning techniques that we have been unable to cover. Also in the Encyclopedia of NMR are several articles on NMR of liquid crystals. 

MeWhinnie, W.R. (1985). In Chemical Bonding and Spectroscopy in Mineral Chemistry (F.J. Berry and D.J. Vaughan, eds.). London Chapman and Hall. Mehring, M. (1983). Principles of High Resolution NMR in Solids. New York Springer-Verlag. 

Mehring, M. 1976 High resolution NMR in solids. Berlin Springer-Verlag. 

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