Low-Frequency Modes in Molecular Crystals. IX. Methyl Torsions and Barriers to Internal Rotation of Some Three-Top Molecules

The low-frequency vibrational spectra of (CH3)3CCN, (CH3)3CCCH, (CH3)3CH, (CH3)3SiH, (CH3)3SiF, (CH3)3P, and (CH3)3N have been recorded for these molecules in both the gaseous and solid states. Both the degenerate E and nondegenerate A2 torsional modes were observed for the molecules in the solid state. The effects of condensation on the torsional modes have been obtained from the far-infrared spectra of the molecules in the gaseous state. The importance of top–top interaction in the torsional potential energy has been demonstrated. The calculated harmonic barriers are 4.31, 4.10, 3.94, 2.49, 2.25, 3.58, and 4.35 kcal/mole for the (CH3)3CCN, (CH3)3CCCH, (CH3)3CH, (CH3)3SiH, (CH3)3SiF, (CH3)3P, and (CH3)3N molecules, respectively, in the solid state. The top–top interaction values ranged from 0.8 kcal/mole for tertiary butylacetylene to 0.3 kcal/mole for isobutane, with an average value of 0.5 kcal/mole.