This paper presents quasiclassical trajectory cross sections and rate constants for the H + H2O → OH + H2 reaction, including a detailed study of reagent normal-mode and local-mode excitation effects, and of product state energy partitioning. The potential surface used is based on a fit to an accurate ab initio calculation. The quasiclassical trajectory calculation used a classical perturbation theory method to define the initial H2O normal-mode and local-mode eigenstates, with the normal-mode representation used to describe the lower energy eigenstates and the local-mode representation used to describe OH stretch overtones having several quanta of excitation. The resulting ground-state thermal rate constants are compared with experimental measurements and found to agree to within their respective uncertainties. Good agreement is also found in comparing our OH product vibration/rotation distributions with the results of recent laser photolysis studies. In studying the influence of initial vibrational excitation, we find substantial rate constant enhancements when any of the three vibrational normal modes of H2O are excited, with most of that enhancement due to a reduction in activation energy which comes from a lowering in the effective cross section threshold energy. The efficiency of this lowering, i.e., the ratio of threshold change to vibrational excitation energy, is highest for the bend mode and lowest for the asymmetric stretch. Our studies of local-mode excitation consider specifically the effect of exciting OH stretch overtones on the reaction rate constant. Most of the emphasis is on the reaction H + HOD → H2 + OD where we compare the reaction rate constant when OH is excited with five quanta to that when the nonreacting OD is excited with the same amount of energy. We find that, although the rate constant is significantly enhanced even when the "wrong" bond is excited, the enhancement is 10-103 larger when the bond being broken is initially excited.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry