ANHARMONIC VIBRATIONAL COUPLING BETWEEN $2\nu_{3}$ AND $\nu_{1}+\nu_{3}$ IN METHANE

$^{1}$K. Fox, G. W. Halsey, S. J. Daunt, W. E. Blass and D. E. Jennings, J. Chem. Phys, 72, 4657 (1980).Author Institution:Our interest in this problem was sparked by reports of anomalous intensity effects in the so-called $2\nu_{3}$ band of $CH_{4}$.$^{1}$ Model force-field calculations predict a substantial off-diagonal matrix element coupling the states $2\nu_{3}$ and $\nu_{1}+\nu_{3}$ of $CH_{4}$ and model intensity calculations with transferred polar tensors predict $2\nu_{3}$ and $\nu_{1}+\nu_{3}$ to have similar intensities in contrast with the observation that only one reasonably intense band is found in the infrared spectrum. The predictions of our model calculations can be brought into good agreement with the experimental observations if we assume that the second derivatives of the electric dipole moment, $\partial^{2} \mu/\partial q_{1} \partial q_{3}$ and $\partial^{2} \mu/\partial q_{3}^{2}$ are important. Our conclusion is supported by ab initio calculations