Correlation Effects and Molecular Tumbling in NMR Studies of Solid bgr;hyphen;(CH3)4Si

摘要

The bgr; phase of high purity samples (99.9percent; min) of tetramethylsilane (TMS), (CH3)4Si, was investigated between 77deg;K and its melting point (174deg;K) employing static and rotating frame proton spinhyphen;lattice relaxation (T1andT1p, respectively) measurements. Minima in theT1curve near 90deg;K and inT1pat 171deg;K correspond to methyl group reorientation and overhyphen;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 140deg;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 spinhyphen;lattice relaxation above sim;140deg;K is associated with the onset of overhyphen;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.