Vapor pressure isotope effects in benzene--cyclohexane systems. III. Theoretical analysis

The data of Jakli, Tzias, and Van Hook (preceeding paper) on vapor pressure isotope effects in the series of deuterobenzenes, solutions of isotopic isomers of benzene and cyclohexane, and mixtures of benzene and cyclohexane are analyzed in terms of the theory of isotope effects in condensed phase systems using a cell model in the quasiharmonic approximation. The single largest contribution to the VPIE's arises from the shift in the CH stretching force constant on condensation but other effects, including external internal coupling and the volume dependence of condensed phase internal frequencies, are also important. Molar volume isotope effects are demonstrated to be of critical importance in arriving at an understanding of the excess thermodynamic properties of solution