Quantum wave packet and quasiclassical trajectory studies of OH+CO: Influence of the reactant channel well on thermal rate constants

We study the OH+CO-->H+CO2 reaction with both six-dimensional quantum wave packets (QM) and quasiclassical trajectories (QCT), determining reaction probabilities and thermal rate constants (or coefficients), and studying the influence of the reactant channel hydrogen-bonded complex well on the reaction dynamics. The calculations use the recently developed Lakin-Troya-Schatz-Harding (LTSH) ground electronic state potential energy surface, along with a modified surface developed for this study (mod-LTSH), in which the reactant channel well is removed. Our results show that there can be significant differences between the QM and QCT descriptions of the reaction for ground-state reactants and for energies important to the thermal rate constants. Zero-point energy violation plays an important role in the QCT results, and as a result, the QCT reaction probability (for ground-state reactants and zero impact parameter) is much higher than its QM counterpart at moderate to low reagent translational energies. The influence of the reactant channel well in the QCT results is to enhance reactivity at moderate energies and to suppress reactivity at the very lowest collision energies. The QM results also show the enhancement at moderate energies but, while the very lowest translational energies cannot be adequately converged, they do not indicate any tendency toward suppression as energy is reduced. QCT calculations for excited rotational states of the reactants show that the suppression of reactivity associated with the reactant channel well is less important when the reactants are rotating, and as a result, the influence of the reactant channel well on the thermal rate coefficients is relatively small, being important below 200 K. Our results indicate that there still remain important discrepancies between experiment and theory in this low temperature regime and that further improvements of the potential are needed. (C) 2004 American Institute of Physics.