Molecular dynamics simulations of fluorinated ethanes

Differences in thermodynamic properties are investigated by means of constant pressure-constant temperature molecular dynamics simulations for three isomeric pairs of fluorinated ethanes: CHF2CHF2 (HFC-134) and CF3CH2F (HFC-134a); CF3CH3 (HFC-143) and CHF2CH2F (HFC-143a); and CH2FCH2F (HFC-152) and CHF2CH3 (HFC-152a). These semi-rigid molecules have interaction centres at the atomic sites and internal rotation about the C-C bond. Intermolecular interactions consist of repulsion-dispersion and Coulombic parts. Molecular dynamics simulations were performed at the normal boiling points and the accord between simulated and experimental values of the potential energy and molar volume is very good. A great difference was found between the repulsion-dispersion and Coulombic contributions to the potential energy for every isomeric pair. A detailed analysis of the repulsion-dispersion and Coulombic interactions was carried out and was discussed in relation to the thermodynamic properties. A comparison of the radial distribution functions, autocorrelation functions and self-diffusion coefficients for these substances is also presented.