Intra and intermolecular structure in the condensed phases of ethylene, ethane and carbon dioxide by neutron diffraction

Abstract

The distinct coherent scattering cross-sections for deuterated ethylene (C₂D₄) at 11·5 bar and 0·00911 mol Å^-3, for deuterated ethane (C₂D₆) at 9·5 bar and 0·00996 mol Å^-3 and for carbon dioxide (CO₂) at 8·5 bar and 0·01597 mol Å^-3 in their liquid phases have been obtained at 220K. These functions have been successfully separated into their intra and intermolecular contributions. The molecular structure of each liquid has been compared with the corresponding intramolecular configuration in both the solid and the gas phases. Intermolecular structure functions, i _m (Q), which contain information about intermolecular correlations, have been extracted. For CO₂(l), the results have been compared with those obtained from molecular dynamic simulations and a reference interaction site model. The intermolecular potential proposed by Böhm et al. gives a good representation of CO₂ liquid near the triple point. However orientational correlations between neighbouring molecules near the triple point, which disappear with increase of temperature, are not described by the model. The molecules in solid ethylene and ethane are packed loosely and the results show that there is only slight relaxation of the structure on melting. The correlation functions for ethylene and ethane have been compared with X-ray diffraction measurements analysed using scattering factors for -CH₂ (in C₂H₄) and -CH₃ (in C₂H₆) groups combined with different interaction models to obtain carbon-carbon intermolecular distribution functions.