Low Frequency Vibrations in Molecular Solids. XII. Low Frequency Modes and Barriers to Internal Rotation of Some Dimethyl Compounds of Elements in Group VI

Abstract

The far infrared spectra of solid CH₃OCH₃, CD₃OCH₃, CD₃OCD₃, CH₃SCH₃, CH₃SeCH₃, CH₃TeCH₃ have been recorded between 350 and 33 cm⁻¹. The B₂ and A₂ torsional frequencies have been assigned at 268 and 244, 245, and 208, 207 and 182, 247 and 210, 207 and 175, and 185 cm⁻¹ for the above molecules at liquid nitrogen temperatures. The barriers calculated from these assignments are 3.50, 3.48, 3.39, 3.22, 2.40, and 2.27 kcal/mole, respectively. The lattice bands observed in the infrared spectra of the isotopic dimethyl ethers have been assigned to librational and translational motion on the basis of the deuteration shifts. The infrared spectra of the sulfide and selenide in the region of the lattice frequencies are similar and a tentative assignment of these bands to librational and translational motions is presented based on the expected shift factors. The infrared lattice spectrum of dimethyl telluride is also presented.