Low-Frequency Vibrations in Molecular Solids. XVIII. Torsional Modes and Barriers to Internal Rotation of Some Isopropyl Compounds

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Abstract
Far-infrared spectra of solid (CH3)2CHCl, (CD3)2CHCl, (CH3)2CHBr, (CD3)2CDBr, (CH3)2CHI, (CD3)2CHI, (CH3)2CHCN, (CH3)2CHOH, and (CD3)2CDOD have been studied from 33 to 500 cm−1. The A′ and A″ torsional modes have been observed and assigned at 248 and 283, 178 and 208, 256 and 272, 189 and 205, 246 and 253, 190 and 198, 230 and 242, 237 and 277, and 169 and 194 cm−1, respectively, for the above molecules. From these assigned frequencies, the torsional barriers were calculated to be 4.48, 4.54, 4.51, 4.77, 4.11, 4.73, 3.63, 4.18, and 3.94 kcal/mole, respectively. These barrier values are compared to those obtained for some similar compounds and certain trends are noted. Some of the lattice frequencies have been assigned to librational and translational motions on the basis of the isotopic shifts with deuteration.