Theoretical Kinetics Predictions for NH₂+ HO₂

Recent modeling studies of NH₃ oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies between existing literature mechanisms. In this study high level theoretical kinetics predictions have been obtained for the reaction of NH₂ with HO₂, which has previously been highlighted as an important reaction with high sensitivity and high uncertainty. The potential energy surface is explored with coupled cluster calculations including large basis sets and high-level corrections to yield high accuracy (∼0.2 kcal/mol) estimates of the stationary point energies. Variational transition state theory is used to predict the microcanonical rate constants, which are then incorporated in master equation treatments of the temperature and pressure dependent kinetics. For the radical-radical channels, the microcanonical rates are obtained from variable reaction coordinate transition state theory implementing directly evaluated multireference electronic energies. The analysis yields predictions for the total rate constant as well as the branching to the NH₃ + O₂, H₂NO + OH, and HNO + H₂O channels. Rate constants are also reported for the H₂NO + OH reaction as they arise naturally from the analysis. The rate constant and branching fraction determined in this work for the NH₂ + HO₂ reaction deviate significantly from values used in most previous modeling studies. The fact that the main product channel is chain terminating, rather than propagating, has strong implications for modeling NH₃ ignition and oxidation, in particular at intermediate temperatures and elevated pressure.