Theory of intermolecular pair correlations for molecular liquids. Applications to the liquids carbon tetrachloride, carbon disulfide, carbon diselenide, and benzene

Abstract

The theory of molecular liquids based on the reference interaction site model (RISM) equation is used to describe the intermolecular structures of the liquids carbon tetrachloride, carbon disulfide, carbon diselenide, and benzene. The theory assumes that the equillibrium correlations between molecules in a dense fluid are determined primarily by the short range, harshly repulsive portions of the intermolecular potential. These repulsive interactions are modeled as hard core interactions by assuming molecules are composed of overlapping hard spheres which are rigidly fused together. The centers of the spheres are located at the positions of the nuclei in the molecules. As a result, the model hard core interactions depend on the molecular orientations. The equilibrium pair correlations produced by these interactions are determined from the solutions of the RISM integral equation. This is an approximate (but accurate) equation for the pair correlation functions. The molecular structure factors S(k) for the above mentioned liquids have been calculated from the theory. The only adjustable parameters in the theory are the diameters of the one or two distinct atomic spheres on a molecule. With physically reasonable values for the diameters, the theoretical results for S(k) agree closely with the results obtained from neutron or x-ray scattering experiments. From the r-space representation of the pair correlation functions, the theory leads to an unambiguous interpretation of the intermolecular structures of the different liquids. For liquid carbon tetrachloride, it is found that the CCl4 molecules form an interlocking structure which gives rise to important orientational correlations between molecules in the liquid. Nearest neighbors in both the liquids carbon disulfide and carbon diselenide tend to be aligned parallel to one another. In liquid benzene, nearest neighbors tend to be perpendicular to each other. These interpretations are discussed in detail. Suggestions are made for future experimental work. Comments are made concerning the usefulness of the RISM equation.