Influence of the angle-dependent part of the dispersion forces on the thermodynamical and spectral properties of gaseous SF₆

Abstract

Analytical expressions for the electrostatic, dispersion and induction interaction between an octahedral molecule and an atom, a linear, tetrahedral, or octahedral molecule are deduced from previous work of the present authors concerning the tetrahedral molecules. This allows the calculation of the second pressure and dielectric virial coefficients, the spectral moments of the pressure induced far infra-red absorption band and the intermolecular mean squared torque for a gas of molecules belonging to the O_h group. Numerical applications to SF₆ gas show that the anisotropic dispersion interaction contribution plays a dominant role in the virial coefficients but not in the spectral moments. It follows that the order of magnitude of the hexadecapole moment for SF₆, previously determined by Rosenberg and Birnbaum from the spectral moments is practically unaffected by consideration of the anisotropic dispersion interaction. In contrast, the calculated values of the virial coefficients are modified significantly by this interaction. This removes the strong discrepancy (by one order of magnitude) appearing between the hexadecapole moment values of SF₆ obtained from the induced far infra-red measurements and that from the virial coefficients when only the electrostatic interaction are considered.