New Aspects of Spin Diffusion and Cross Relaxation in Solid-State NMR

Theoretical models for motionally driven spin diffusion are compared and applied to spins S = (1/2) and S = 1. Their predictions are quantitatively corroborated by experiments on a single crystal of deuterated biphenyl. In this system, a molecular flip process drives spin diffusion and makes it strongly temperature dependent. In the second part, it is 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 spins S = 1. This observation explains why, in solids, the relaxation time of quadrupolar order, T 1Q , is often much shorter than T 1 .