Theoretical Kinetics Predictions for Reactions on the NH₂O Potential Energy Surface

Recent modeling studies of ammonia oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies among the existing literature mechanisms. In this study, high-level theoretical kinetics predictions have been obtained for reactions on the NH₂O potential energy surface, including the NH₂ + O, HNO + H, and NH + OH reactions. These reactions have previously been highlighted as important reactions in NH₃ oxidation 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 2σ uncertainty) 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 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 constants as well as the branching ratios. We find that the NO + H₂ channel contributes 10% of the total NH₂ + O flux at combustion temperatures, although this channel is not included in modern NH₃ oxidation mechanisms. Modeling is used to illustrate the ramifications of these rate predictions on the kinetics of NH₃ oxidation and NO formation. The present results for NH₂ + O are important for predicting the chain branching and formation of NO in the oxidation of NH₃ and thermal DeNO.