Infrared spectra, rotational correlation functions, and intermolecular mean squared torques in compressed gaseous methane

Abstract

Extensive measurements of the methane ν₃ and ν₄ fundamental vibration–rotation bands in CH₄–He mixtures and the ν₃ band in CH₄–He, CH₄–N₂, and CD₄–He mixtures have been carried out in infrared absorption at 295 °K to pressures of 3000 atm. Some profiles of the ν₃ band in CH₄–Ar mixtures and in pure CH₄ have also been obtained. Rotational correlation functions, band moments, and intermolecular mean squared torques have been determined from the ν₃ band profiles. Theoretical calculations of the mean squared torque due to anisotropic multipolar, induction and dispersion interactions have been carried out. The theoretical and experimental torques are in order-of-magnitude agreement for the CH₄–N₂ and CH₄–CH₄ systems; for CH₄–He, CD₄–He, and CH₄–Ar the theoretical values are two to three orders of magnitude too small to account for the experimental values, indicating that in these cases the dominant contribution to the torques is given by the anisotropic overlap forces.