NMR Relaxation Studies of Solid Hexamethylethane and Hexamethyldisilane

Abstract

Pulsed NMR studies in the static and rotating frames (T1 and T1ρ, respectively) of the proton spin-lattice relaxation time were carried out on solid hexamethylethane (CH3)3CC(CH3)3 and hexamethyldisilane (CH3)3SiSi(CH3)3 between 77°K and the melting point. The phase transition in each of the solids is accompanied by a discontinuity in the T1 and T1ρ plots. In the low-temperature phase of HMDS, the proton relaxation produced by reorientation of the CH3 group about its threefold axis (C3 rotation) is resolved from that produced by reorientation of the molecule about its threefold axis (C3′ rotation). The corresponding activation energies are 1.56±0.08 and 7.50±0.40 kal/mole. (Actually, the results for HMDS are more in keeping with the CH3 group motional narrowing and relaxation being caused by quantum mechanical torsional-state effects than by classical rotation.) In HME the effects of the two types of rotation are combined and have an apparent activation energy of 3.10±0.15 kcal/mole. The minimum T1 and T1ρ values were calculated for these motions by means of the BPP and extended BPP theories and are in good agreement with our experimental results. In the high-temperature phase of both solids, self-diffusion as well as over-all molecular reorientation or tumbling governs the proton relaxation. The activation energy for tumbling was found to be ·1.5 kcal/mole for HME and 2.20±0.10 kcal/mole for HMDS. Torrey's diffusion theory is employed to analyze the contribution to T1ρ caused by self-diffusion, and the activation energy for the process is estimated to be 18±2 and 8.7±0.4 kcal/mole for HME and HMDS, respectively. The properties of these two compounds are discussed briefly in relation to the NMR and thermodynamic studies of them.