INDIVIDUAL SPECTRAL DENSITIES AND MOLECULAR-MOTION IN POLYCRYSTALLINEHEXAMETHYLBENZENE-D18

Methods are described for obtaining the orientation dependence of indi vidual motional spectral densities, J1(omega0) and J2(2omega0), from d euterium spin relaxation experiments on polycrystalline materials. Spe ctral density measurements provide detailed information in a motional regime too fast to be studied by the two-dimensional (2D) exchange met hod. Their potential as a source of detailed kinetic and geometric inf ormation is illustrated for hexamethylbenzene-d18 (HMB). The relaxatio n behavior of HMB cannot be explained exclusively by six-site jumps ar ound the C6upsilon axis. Agreement between the experimentally determin ed spectral densities and simulations is improved if the methyl rotati on is explicitly included. At ambient temperature the experimental dat a are best fitted with the simultaneous jump rates, k6=3.85 X 10(8) s- 1 and k3=5.0 X 10(11) s-1. This is significantly different from the ra te determined using a simple six-site jump model, k6=3.9 X 10(9) s-1. Geometric distortions of the methyl rotation axes can account for the observed motionally averaged electric field gradient tensor. When thes e distortions are included in analysis of the spectral density data, t here is a small, but significant, improvement in the fit. k3 is unchan ged and the best fit k6 is reduced to 2.2 X 10(8) S-1, with distortion s out of plane by delta=2.5-degrees and in plane epsilon=epsilon'=1.20 2.