Understanding and Representing the Distinct Kinetics Induced by Reactive Collisions of Rovibrationally Excited...

Rovibrationally excited ephemeral complexes AB^**, formed from the association of two molecules A + B, are generally considered to undergo collisions only with an inert bath gas M that transfer energy-inducing termolecular association reactions A + B (+M) → AB (+M). Recent studies have demonstrated that reactive collisions of AB^** with a third molecule C-inducing chemically termolecular reactions A + B + C → products-can also be significant in combustion and planetary atmospheres. Previous studies on systems with reactive collisions have primarily focused on limited ranges of reactive collider mole fraction, X_C, and pressure, P, specific to the chosen application. Yet, it remains to be established how such systems, and the rate constants of their emergent phenomenological reactions, behave over the wide X_C and P ranges of potential interest-a gap in the present understanding that has impeded the development of broadly applicable rate laws and general treatment of such systems in kinetic modeling. Here, we present results from master equation calculations for HO₂^** formed from H + O₂ and its reactions with H to advance understanding and explore representations of systems with reactive colliders across wide ranges of X_C and P. With regard to understanding, we demonstrate that reactive collisions can both (1) increase the overall rate of conversion of reactants to products and (2) alter the branching ratio among final products. With regard to representations in kinetic models, we find that rate constants of all emergent phenomenological reactions-termolecular association A + B (+M), chemically termolecular A + B + C, and bimolecular AB + C-exhibit a rich X_C and P dependence. We also present analyses to explore the existence of a unique phenomenological representation (or lack thereof) and assess ways for the distinct effects of reactive collisions to be represented in kinetic models.