Low Temperature and High Pressure Far Infrared Spectra of Condensed Phases: I. Mono- and Dimethylcyclohexanes

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Abstract
The infrared spectra (60 to 400 cm−1 and 800 to 1100 cm−1) of methylcyclohexane and of all the dimethylcyclohexanes except the 1,1-isomer were obtained with a Michelson interferometer at temperatures between ambient and −150°C and—in the latter wavenumber region—compared with spectra gathered earlier on the same materials maintained in a diamond anvil cell under pressures ranging from ambient to about 40 kb. Most of the spectral bands changed similarly with increasing pressure or decreasing temperature, but there were exceptions ascribable to conformational changes. The intensities of bands associated with analogous modes of vibration varied similarly in these isomers; i.e., they either increased or decreased so that, conversely, this behavior could be used in their assignment to conformational modes. The frequency shifts of corresponding bands between cis and trans isomers were in the same direction for the 1,2- and 1,4-isomers, but opposite for the 1,3-isomers, reflecting the greater stability of the trans isomers in the former case, but of the cis isomer in the latter.