Theory of inverse isotope effects

The inverse isotope effect in isotope exchange equilibria is attributable to the temperature-dependent contributions from various vibrationally excited states. The inverse effects are predictable from the single-term G(u)-approximation. Cross-over and other anomalies of the temperature-dependence of isotope-exchange equilibria are discussed in terms of the finite orthogonal expansion of the reduced partition function ratio. The inverse vapor pressure isotope effect in polyatomic molecules is discussed within the framework of the simple cell model. The inverse effect is due to the isotopic difference in the shifts of zero-point energy of internal vibrations upon condensation. The zero-point shift on condensation of nonpolar molecule is related to the cartesian dipole moment derivatives. The latter are calculated by means of a self-consistent field molecular orbital theory. A Medium Cluster Model is proposed as an extension of the cell model of liquid as a basis of the vapor pressure isotope effect. The liquid phase is represented by a cluster consisting of a central molecule and several shell molecules