Effect of Vibrational Anharmonicity on the Isotopic Self-Exchange Equilibria H2X+D2X=2HDX

The anharmonicity correction to the zero-point energy change in the triatomic dihydride isotopic self-exchange equilibria, H2X+D2X=2HDX, is discussed. It is shown that the inclusion of the usually ignored term G0 in the expression for the anharmonic correction to the zero-point energy (G0 + 14Σxij) causes the anharmonicity correction to the zero-point energy change to be quite small for the cases considered (X is O, S, Se), but when this term is ignored the correction is found to be nonnegligible. Equilibrium constants are calculated, and it is found that the agreement between theory and experiment is considerably improved by including the G0 factor.