Third-order potential constants of bent XY 2 molecules

Abstract

In pursuit of a method for predicting the cubic constants and the vibration-rotation interaction constants α of polyatomic molecules, the physical significance of the third-order constants (the cubic coefficients of the expansion of the internuclear potential function in terms of the internal coordinates) of the bent XY 2 molecule has been studied. An anharmonic potential function of the valence-force type (which includes general quadratic force constants and a third-order coefficient of the bond-stretching displacements) is supplemented by a function which depends on the YY distance as an empirical model, and the third-order constants are considered in relation to this function. The calculations which make use of several empirical functions for the hydrogen-hydrogen and neon-neon interactions taken from the literature show that the signs (negative in most cases) and the orders of magnitude of the third-order constants of H 2O, D 2O, H 2S, H 2Se, D 2Se, SO 2 and OF 2 can be accounted for fairly satisfactorily by this model. This model is also able to make correct order-of-magnitude estimations for many of the cubic and the α constants of these molecules. In particular, the estimates of the α A 2 and α B 2 constants of H 2O, D 2O, SO 2 and OF 2 appear in better agreement with the corresponding experimental values than those based on the valence-force model. The predictions based on the non-bonded interaction model, however, are found to be poor in some cases, for example, for the ⨍ rr′α and α A 1 constants of OF 2.