Effects of reactant rotational excitation on reactivity: perspectives from the sudden limit.

Excitation of reactant rotational degrees of freedom is known to influence reactivity in bimolecular reactions. In this work, this effect is examined for several prototypical activated atom-diatom and atom-triatom reactions through exact quantum scattering calculations on accurate ab initio potential energy surfaces. To rationalize these mode-specific effects, the recently proposed sudden vector overlap model is extended to include rotational motions of reactants. Specifically, the enhancement of reactivity is attributed to their coupling with the reaction coordinate at the transition state, as quantified by the alignment between the corresponding normal mode vectors. In addition, a Franck-Condon model is introduced to predict the effect of reactant rotational excitation for reactions in which the reactant rotations are decoupled from the reaction coordinate.