Vapor pressure isotope effects in liquid methylene difluoride

Abstract

The vapor pressures of the isotopic methylene difluorides '*H2F2, I3CH2F2, and '%D2F2 have been measured at temperatures between 149.36 and 244.82 K by differential manometric techniques in a precision cryostat. Throughout the whole temperature range of the measurement, P(13CHzF2) > P(I2CH2F2) > P(CD2F2). The data are best represented by T In cfc/fg) = -(31.64 f 1.97)/T- (0.4069 f 0.0107) for the I2C/l3C effect and by Tln cf,/f,) = (632.26 f 97.62)/T- (19.175 f 1.016) - (0.0532 f 0.0025) T for the H/D effect. The vapor pressure of the natural abundance methylene difluoride is given by log P(Torr) = 7.1990 - 842.31/[t (C) + 246.811. The normal H/D vapor pressure isotope effect (VPIE) in liquid methylene difluoride is due to the zero-point energy shift upon condensation of the deuterio species being greater than that of the protio species, a fact which is also found for the H/D VPIEs in liquid methyl fluoride and fluoroform. The H/D effect in methylene difluoride is unusually higher than those found in methyl fluoride and fluoroform due to an enhanced perturbation of the C-H stretching modes by a relatively large intermolecular force in liquid methylene difluoride. Results of the normal-coordinate analyses of the cell model are presented. Temperature dependency of the external-internal interaction force constants is a necessary requirement for the satisfactory reproduction of the observed H/D and l2C/l3C VPIEs and spectroscopic data on the liquid.