H/D and 12C/13C vapor pressure isotope effects in liquid methylene difluoride studied by density functional theory

Abstract

H/D and 12C/13C vapor pressure isotope effects (VPIE’s) in liquid CH2F2 were studies at the mPW1PW91/6-31++G(d) level of theory. The optimized monomer and CH2F2 molecules surrounded by other CH2F2 molecules in optimized CH2F2 clusters were used as model molecules of CH2F2 vapor and liquid CH2F2, respectively. The calculations qualitatively reproduced the normal hydrogen VPIE and the inverse carbon VPIE, although they overestimated the former and underestimated the latter at 170 K. The number of CH···F(C) intermolecular interactions between hydrogen and fluorine atoms of neighboring molecules in the liquid phase and their strength were required to be temperature-dependent to explain the experimental temperature dependences of the VPIE’s.