The structure of liquid carbon dioxide and carbon disulfide

We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO₂ and CS₂ at a density of about 10 molecules/ nm3. Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density ρ, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO₂; for CS₂ we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function gSS. Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO₂ and CS₂ do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS₂ are changed to those of CO₂, allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.