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.