Low-Frequency Vibrations of Molecular Solids. V. Globular Molecules—(CH3)3MCl and (CH3)3MBr

The far-infrared and Raman spectra of solid (CH3)3CCl, (CD3)3CCl, (CH33SiCl, (CH3)3GeCl, (CH3)3CBr, (CH3)3SiBr, and (CH3)3GeBr have been obtained over a range of temperatures. For (CH3)3CCl, (CD3)3CCl, (CH3)3SiCl, and (CH3)3CBr torsional vibrations were observed, and from the frequencies, barriers to internal rotation of the methyl groups were calculated. The barrier of 4.51 kcal/mole for tertiary-butyl chloride (4.82 for the d9 compound) is consistent with that reported earlier for the fluoride. The 3.90-kcal/mole barrier calculated for the tertiary-butyl bromide is somewhat lower than expected. A barrier of 2.63 kcal/mole was found for the trimethylchlorosilane molecule. Intermolecular fundamentals were observed for all compounds in the series, and the higher-frequency modes observed in the infrared spectra were assigned as librational fundamentals whereas the lower-frequency Raman lines have been assigned as optical translations. The frequency shifts for these intermolecular fundamentals are discussed in terms of the increased intermolecular forces for the higher molecular weight compounds. Although the “heavy” members of the series are globular in shape, no phase transitions were observed and the lack of more than one crystalline form for these solids is believed to be the result of the relatively large intermolecular forces.