Hydrogen bonding and the dipole moment of hydrofluorocarbons by density functional theory

Recent measurements of the dielectric permittivity of hydrofluorocarbons in the liquid phase have allowed calculation of the dipole moments in a liquid environment. These values were based on Kirkwood theory, and were significantly greater than the corresponding gas phase dipole moments. In order to understand some features suggesting possible hindered rotation of the molecules in the liquid, density functional and self-consistent-reaction-field calculations for a series of HFC molecules including CHF2CF3 (HFC-125), CH2FCF3 (HFC-134a), CH3CF3 (HFC-143a), CH2F2 (HFC-32) and CHF2CH3 (HFC-152a) are reported. Particular emphasis has been given to the calculation of dimerisation energies, rotational potentials, polarisabilities and dipole moments. We discuss hydrogen bonding in hydrofluorocarbon dimers and the relationship between the structure and charge distribution of the dimers and the dipole moment in the liquid predicted by relative permittivity measurements. For HFC-32 we have calculated the average dipole moment in small clusters (n = 2-10). The structure of the clusters has been determined by density functional theory optimisations (n = 2-6) and Monte Carlo simulations (n = 2-10). The average dipole moment of the HFC-32 decamer is 2.35 D, which represents a 17% increase relative to the free monomer (2.0 D). We find that the enhancement of the monomer dipole induced by hydrogen bonding in HFC-32 clusters is much less pronounced in comparison with the considerable increase (50%) observed in water clusters.