The structure of fluid trifluoromethane and methylfluoride

We present hard x-ray and neutron diffraction measurements on the polar fluorocarbons HCF₃ and H₃CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto inversion scheme has been used to deduce the site-site potentials underlying the measured partial pair distribution functions. The orientational correlations between adjacent fluorocarbon molecules - which are characterized by quite large dipole moments but no tendency to form hydrogen bonds - are small compared to a highly polar system like fluid hydrogen chloride. In fact, the orientational correlations in HCF₃ and H₃CF are found to be nearly as small as those of fluid CF₄, a fluorocarbon with no dipole moment.