Gl. Hoatson et al., INDIVIDUAL SPECTRAL DENSITIES AND MOLECULAR-MOTION IN POLYCRYSTALLINEHEXAMETHYLBENZENE-D18, The Journal of chemical physics, 100(7), 1994, pp. 4756-4765
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.