Molecular reorientation and self-diffusion in the plastic solid hexamethyldisilane studied using nuclear magnetic resonance and radiotracer techniques

Abstract

Proton nuclear magnetic relaxation times, T₁, T_1ρ and T₂, linewidth and radiotracer self-diffusion coefficients have been measured as a function of temperature in high purity samples of hexamethyl-disilane. The activation enthalpies for methyl group reorientation and molecular reorientation about the Si—Si axis in the low temperature phase and molecular tumbling in the high temperature plastic phase were determined from the T₁ and T_1ρ measurements. These values are in general agreement with the results of a similar, but less extensive, n.m.r. study made by Albert et al. The Torrey theory was found to give the most satisfactory analysis of the translational self-diffusion contribution to the relaxation times in the plastic phase. The diffusion coefficients obtained yielded curved Arrhenius plots and a single activation enthalpy does not describe this motion. Similar curvature was not found in the radiotracer measurements. However, these were made over a smaller temperature range. The difference between the radiotracer and n.m.r. measurements is slightly higher than expected from a consideration of correlation effects and this could be due to self-diffusion in hexamethyldisilane proceeding predominantly by a mechanism which involves relaxed vacancies. The results are compared with those for other plastic crystals where pulsed n.m.r. and radiotracer data are available.