Low-Frequency Vibrations of Molecular Solids. XI. Torsional Frequencies and Barriers to Internal Rotation of Some Single Methyl Rotors in the Solid State

The torsional frequencies of $${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Cl},{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br},{\mathrm{CD}}_3{\mathrm{CD}}_2\mathrm{Br},{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{I},$$ $${\mathrm{CD}}_3{\mathrm{CH}}_2\mathrm{I},{\mathrm{CH}}_3{\mathrm{CHOCH}}_2,{\mathrm{CH}}_3{\mathrm{CHSCH}}_2,{\mathrm{CH}}_3\mathrm{COCl},{\mathrm{CD}}_3\mathrm{COCl},\mathrm{and}{\mathrm{CH}}_3\mathrm{COBr},$$ have been observed in the solid state and the measured values are: 278, 276, 204, 258, 198, 223, 250, 167, 124, and 168 cm⁻¹, respectively. The torsional ferquencies in the solid state are compared with values obtained by both infrared and microwave techniques for the gaseous molecules. The gas–solid phase shifts of the torsional and skeletal bending frequencies are discussed in terms of the intermolecular forces in the crystals. Pronounced factor‐group splitting was observed in the far‐infrared spectra of the solid ethyl halides and these splittings are also discussed in terms of the intermolecular potential field. Lattice spectra were recorded for all the molecules studied in the crystalline state, and the frequencies are assigned to librational or translational modes on the basis of isotopic shift factors in the cases for which such data are available.