Spin-lattice relaxation in solids

Abstract

Spin-lattice relaxation times ( T 1) contain information about molecular motion. We present a formalism for extracting this information from T 1 values measured for static and spinning polycrystalline solids. General equations are developed for orientation dependent line positions (frequencies) and values of T 1. The correlation functions in the general T 1 expressions are evaluated for various jump and diffusive models of motion, assuming an axially symmetric coupling mechanism. Explicit expressions for dipolar and quadrupolar values of T 1 are presented for two- and three-site jumps, and for continuous diffusion about a symmetry axis. It is shown that T 1measurements can readily distinguish between three-site jump and continuous diffusion models of methyl reorientation on the basis of the different angular dependence calculated for T 1 in these two models. In general, T 1 is axially asymmetric for the three-site jump model, resulting in nonexponential relaxation for the spectral lines in the axially symmetric powder pattern.