Low-Frequency Modes in Molecular Crystals. VI. Methyl Torsions and Barriers to Internal Rotation of C(CH3)4, C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4

The far-infrared spectra of C(CH3)4, C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4 have been recorded from 300 to 33 cm−1. The f1 torsional modes were observed at 281, 206, 177.5, 134, and 101.5 cm−1 and the a2 torsional modes were assigned to bands at 221, 157, 163.5, 134, and 101.5 cm−1 for the C(CH3)4, and C(CD3)4, Si(CH3)4, Ge(CH3)4, and Sn(CH3)4 molecules, respectively. The corresponding CH3 torsional barriers were calculated to be 4.3, 4.6, 2.0, 1.3, and 0.8 kcal/mole. These barriers are shown to be consistent with those reported for the similar trimethyl molecules but are considerably higher than the barriers previously reported for these molecules from PMR studies.