The structure of fluid trifluoromethane and methylfluoride

We present hard x-ray and neutron diffraction measurements on the polar flu orocarbons HCF3 and H3CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reat to inversion scheme has been used to deduce the site-site potentials underl ying the measured partial pair distribution functions. The orientational co rrelations between adjacent fluorocarbon molecules-which are characterized by quite large dipole moments but no tendency to form hydrogen bonds-are sm all 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.