Infrared Spectra of Some Dimethylcyclohexanes at Very High Pressures

Absorption spectra in the 500–1100 cm−1 region were obtained for cyclohexane, methylcyclohexane, and all the dimethylcyclohexanes except 1,1-dimethylcyclohexane, at pressures estimated to range between ambient and 40 kb. A diamond anvil cell was used to contain the samples. Simple modifications to an RIIC Model FS-720 Far Infrared Interferometer allowed it to operate in this spectral region. All the samples are liquids at ambient pressure and become very viscous before crystallization except trans-1,3-dimethylcyclohexane, which could not be crystallized. It is shown theoretically that high pressure can change the distribution of conformers of some of the isomers, and spectral changes can be interpreted to support this deduction. Just as cyclohexane undergoes a phase change in the solid state, so do both cis- and trans-1,2-dimethylcyclohexane, for their spectra change with increasing pressure. The first crystalline structure formed under increasing pressure from the latter material appears to be rather unstable and to correspond to the biaxial conformer.