Anomalous isotope effect in Ar-H2S versus the normal effect in Ne-H2S

Substitution of deuterium for hydrogen in the weakly bound cluster Ar-H2S is known to produce an anomalous effect in the ground vibrational state's average rotational constant [B + C]/2 in contrast to deuteration of Ar-H2O. The anomalous effect develops from the vibrational dynamics, and we show in this report that the effect is not likely to be found for deuteration of the Ne-H2S cluster. Ab initio calculations were used to generate a potential-energy surface for Ne-H2S. That surface and a surface derived from it were used in rigid body diffusion quantum Monte Carlo calculations to obtain the rotational constants of the ground vibrational states and thereby determine whether deuterium substitution increases or decreases [B + C]/2. From these results, we show that the exceptional vibrational dynamics of Ar-H2S are a consequence of a surface feature, the extensiveness of a low-energy trough. Though a similar trough is found for Ne-H2S, it is not as extensive. In carrying out these calculations, we achieve an assessment of the role certain surface features and their representations play in vibrational averaging. The calculations also provide a prediction that [B + C]/2 for Ne-H2S, for which there is no spectroscopic value available yet, is within the range of 2344-2493 MHz. (C) 1999 American Institute of Physics. [S0021-9606(99)03101-3].