Correlation Effects and Molecular Tumbling in NMR Studies of Solid β-(CH3)4Si

Abstract

The β phase of high purity samples (99.9% min) of tetramethylsilane (TMS), (CH3)4Si, was investigated between 77°K and its melting point (174°K) employing static and rotating frame proton spin-lattice relaxation (T1 and T1p, respectively) measurements. Minima in the T1 curve near 90°K and in T1p at 171°K correspond to methyl group reorientation and over-all molecular tumbling, respectively. Activation energies and inverse frequency factors for both motions are derived and discussed. Deviations from exponential behavior in the longitudinal relaxation were noted below 140°K and are inferred to be due to a correlation in the relative motion of the protons within each methyl group. The exponentiality of the spin-lattice relaxation above ∼140°K is associated with the onset of over-all molecular tumbling motions. It is suggested that the presence of exponential proton magnetization decays in solids containing methyl groups may sometimes serve as an indication of the presence of reorientations of the molecular frame to which the methyl groups are attached. The possibility that the existence of exponential and nonexponential decays may be used by itself to gain information on interactions and motions in solids, has to be further investigated.