Developments in the Motional Narrowing of the Nmr Spectra of Solids— Microscopic and Macroscopic

An investigation of microscopic motional narrowing of nmrspectra of polycrystalline tetra-, tn-, diand mono-methylammonium chloride is reported. A diversity of rotational motions of the ions and of the methyl groups within the ions is exhibited. The problem of detailed interpretation of the line-narrowing is particularly acute for two of these compounds, and also for solid 1-valine. The proper use of the decreasing second moment of motionally-narrowed nmr spectra as an alternative means of obtaining an activation energy is discussed. The variation of second moment with correlation time has been calculated on the basis of Kubo and Tomita's theory of motional narrowing, and the use of the measured second moments to extract correlation times is described. Conformational motion in solids and the consequent narrowing of their nmr spectra is discussed. Consideration is given to the possibility of a conformational order—disorder process, with special reference to solid cyclobutane. The variation of nmr linewidth with order parameter has been calculated. The propagation of conformational waves analogous to spin waves and librational waves is proposed, and the corresponding quasi-particle characterizing elementary excitation of such motion is called a conformon. The application of macroscopic magic-axis rotation to metallic copper is described. An order of magnitude improvement in precision has been obtained in Knight-shift determination for both copper isotopes. The accuracy of measurement of relative chemical shift for both isotopes in three cuprous halide reference compounds is likewise improved. At high speeds of rotation a residual rotationally-invariant linewidth is observed for both isotopes in metallic copper. This is attributed to the Ruderman—Kittel interaction, and enables this interaction to be determined for lighter metals for which it is ordinarily obscured by magnetic dipolar broadening. Reference is made to the contribution of microscopic molecular motion within a solid to the effectiveness of macroscopic motion in securing nmr spectral narrowing, and examples of different cases are treated.