DIPOLAR INTERACTIONS IN DEUTERON SPIN SYSTEMS .1. SPIN-DIFFUSION

The influence of dipolar interactions on the longitudianl relaxation o f deuteron spin systems is investigated. Spin diffusion rates are eval uated, including approximate rates due to double quantum spin diffusio n and three-spin flip-flop transitions. It is shown that slow molecula r rotations in supercooled liquids do not affect the spin diffusion ra tes significantly provided that the motional correlation times are bel ow the average spin lattice relaxation time T-l which becomes on the o rder of one second close to the glass transition temperature T-g. Howe ver, the broad distribution of deuteron T-l values at T < T-g results in a large effect of spin diffusion upon the long time decay of the lo ngitudinal magnetization in T-l experiments. These effects are estimat ed in terms of a simple model in agreement with recent experiments. It is also shown that the initial decay determining the average rate [T- l(-l)] remains unaffected by spin diffusion. Finally, we show that sma ll amplitude motions on a time scale of 10(-6)-10(-3) s may cause temp erature dependent spin diffusion effects. (C) 1995 American Institute of Physics.