Reaction coordinates for elucidating reaction dynamics with anharmonic couplings

Abstract

Saddle points can be found on the potential energy surface between the reactants and products regions in many chemical reactions. The dynamics occurring in the vicinity of a saddle point play an essential role in the study of reaction dynamics because the reactivity of a trajectory is primarily determined in the saddle region. There are many cases where a well-defined boundary (called reactivity boundary here) between reacting and nonreacting trajectories can be located in the phase space. Locating the reactivity boundary, however, becomes challenging when anharmonic couplings between the reaction coordinate and the vibrational coordinates complicate the reaction dynamics. The aim of this article is to present a review of recent studies that use nonlinear coordinate transformations to disentangle the anharmonic couplings. The reaction coordinates constructed through such nonlinear coordinate transformations are decoupled from the other coordinates as much as possible, thus simplifying analysis of reaction dynamics. Several options are introduced for nonlinear coordinate transformations which should be appropriately chosen by examining the extent of the anharmonic couplings. Under certain conditions, special reaction coordinates constructed by a suitable nonlinear coordinate transformation reveals the existence of a clear reactivity boundary in the phase space even when there are strong anharmonic couplings.