NEW ASPECTS OF SPIN-DIFFUSION AND CROSS-RELAXATION IN SOLID-STATE NMR

Theoretical models for motionally driven spin diffusion are compared a nd applied to spins S = 1/2 and S = 1. Their predictions are quantitat ively corroborated by experiments on a single crystal of deuterated bi phenyl. In this system, a molecular flip process drives spin diffusion and makes it strongly temperature dependent. In the second part, it i s shown how spin diffusion of quadrupolar order degenerates into cross relaxation. A single crystal of partially deuterated durene provides a clear-cut example of cross relaxation in a dipolar coupled pair of s pins S = 1. This observation explains why, in solids, the relaxation t ime of quadrupolar order, T-1Q, is often much shorter than T-1. (C) 19 97 Academic Press.