Theory of phase transitions in solid methanes. XI. Infrared and Raman spectra of the ν3 and ν4 modes in phase II of solid CH4

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Abstract
The infrared and Raman spectra of the ν3 and ν4 modes of intramolecular vibration of methane in phase II have been theoretically studied on the basis of the extended James–Keenan model. Low‐lying vibration–rotation states coupled through the first order Coriolis coupling are obtained. Two kinds of energy level schemes have been worked out, one for the molecules with an orientational order (D2d molecules) and the other for those without it (Oh molecules). The rotational structures (positions and intensities) of the ir spectra have been calculated, and a good agreement with experiment has been obtained, if the fundamental vibration frequencies of the ν3 and ν4 modes are put equal to 3012.1 and 1295.9 cm−1 for Oh molecules and 3007.3 and 1300.8 cm−1 for D2d molecules, respectively. Raman spectra are also treated and their rotational structures are predicted.