X-ray diffraction study and models of liquid ethylene at 106 K

X-ray diffraction data for liquid ethylene near the melting point are presented. The data are analyzed using scattering factors for --CH2 groups which permits extraction of the intermolecular carbon--carbon structure and distribution functions. Atom pair potentials derived from studies on crystalline hydrocarbons are used with the reference interaction site model (RISM) to predict structure and distribution functions for a dense ethylene-like fluid, and these are compared to the curves derived from the diffraction study. Site--site interaction potentials give an adequate description of liquid ethylene, provided that the attractive forces are averaged over the --CH2 groups with repulsive ''bulges'' at the hydrogen positions in the C2H4 molecule. Although --CH2 groups are ''seen'' by x rays as spherically symmetric, the hydrogen atoms have a significant effect on the carbon--carbon correlations in liquid ethylene near the triple point