Probing the gas-phase oxidation of ammonia: Addressing uncertainties with theoretical calculations

The kinetics of the reactions H2NO + O2(3Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(3Σg−), which are, respectively, very sensitive chain-propagation and chain-termination reactions in ammonia kinetic models, have been revisited by means of high-level electronic structure and variational transition state theory calculations with the goal of improving former predictions and the performance of ammonia kinetic models. In addition, the rate constants of the reactions H2NO + O2(3Σg−) → HNO(a˜3A″) + HO2, NH2 + HO2 → H2NO + OH, and NH2 + HO2 → NH3 + O2(1Δg), which take place on excited-state potential energy surfaces and/or yield the electronically excited species HNO(a˜3A″) and O2(1Δg), have been also calculated for the first time in order to assess their importance in ammonia oxidation. We observed that spin contamination and multi-reference character are pronounced in many of the investigated reactions, and these features were handled by performing post-CCSD(T) electronic structure calculations with the W3X-L composite method as well as restricted open shell coupled cluster calculations. Branching ratios were also analyzed, and indicate that the contribution of the electronically excited species HNO(a˜3A″) and O2(1Δg) are of little importance even at very high temperatures; however, we do not preclude an effect of those species at certain conditions that contribute to their yield. The calculated rate constants were implemented in two recent kinetic models to perform jet stirred reactor, rapid compression machine, and flow reactor simulations, concluding that the model predictions are very sensitive to the reactions H2NO + O2(3Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(3Σg−).