Anisotropy of molecular reorientational motions in CD₃H and CH₃D dissolved in various simple fluids

Abstract

In order to study the change occurring in the reorientational motions when passing continuously from the gas to the liquid phase, we have investigated the infra-red band profiles of CD₃H dissolved in rare gases, nitrogen and oxygen, and of CH₃D in methane (CH₄), in the whole liquid range, from the triple to the critical point and along an isobar above the critical pressure. The j diffusion model, which assumes the identity of the reorientational processes of the C ₃ axis and about this axis was first tested on the experimental results. This model appears to be unsatisfactory at high densities and especially in liquefied gases near the triple point of the solvents, except in the case of the solutions in methane where it can still be justified down to the triple point and in the plastic phase. A better approximation is obtained by extending the continued fraction representation of the dipole moment correlation function up to the second order. Thus, the simultaneous description of the experimental parallel and perpendicular band profiles with this second-order approximation enables us to compare the various components of the mean square torque and thus to get an estimation of the anisotropy of the rotational motion. We discuss also the temperature dependence of the mean square torque along the saturation line of the solvents. A comparison is made with the multipole contributions calculated from the perturbation theory.