Far infra-red manifestations of the intermolecular dynamics in compressed gaseous and liquid CClF3

Abstract

The spectra of chlorotrifluoromethane (CClF₃) have been taken in the region 2–100 cm^–1 at pressures of 1.0–147.7 bar, and in the liquid phase. In all cases, the pure rotational envelope is dominant, suggesting that, even in the liquid, the molecular dynamics are those of collision-interrupted almost-free rotation. Mean times between collisions have been estimated by comparing vectorial correlation functions derived from the experimental data with those predicted by McClung's extended J-diffusion model. The higher frequency parts of the bands observed have their origin in collision-induced dipolar absorption, which has been quantitatively treated using equations for multipole-induced dipolar absorption in bimolecular collisions derived from Frost's general theory for symmetric top molecules. Using these equations, an approximate value for the quadrupolc moment (|Q|) of 11 × 10^–40 C m² has been estimated from moderately low pressure data. The value of |Q| [graphic omitted] 6 × 10^–40 C m² obtained in the same way from the liquid and highly compressed gaseous data reflects inter alia the loss of validity of the bimolecular collision model and of the point multipole expansion of the molecular field when the mean distance travelled between collisions is of the order of the molecular van der Waals diameter. This last point is illustrated using a local charge basis for the field in the near neighbourhood of a molecule.