Rate Constant and Branching Fraction for the NH2 + NO2 Reaction

The NH2 + NO2 reaction has been studied experimentally and theoretically. On the basis of laser photolysis/LIF experiments, the total rate constant was determined over the temperature range 295–625 K as k1,exp(T) = 9.5 × 10–7(T/K)−2.05 exp(−404 K/T) cm3 molecule–1 s–1. This value is in the upper range of data reported for this temperature range. The reactions on the NH2 + NO2 potential energy surface were studied using high level ab initio transition state theory (TST) based master equation methods, yielding a rate constant of k1,theory(T) = 7.5 × 10–12(T/K)−0.172 exp(687 K/T) cm3 molecule–1 s–1, in good agreement with the experimental value in the overlapping temperature range. The two entrance channel adducts H2NNO2 and H2NONO lead to formation of N2O + H2O (R1a) and H2NO + NO (R1b), respectively. The pathways through H2NNO2 and H2NONO are essentially unconnected, even though roaming may facilitate a small flux between the adducts. High- and low-pressure limit rate coefficients for the various product channels of NH2 + NO2 are determined from the ab initio TST-based master equation calculations for the temperature range 300–2000 K. The theoretical predictions are in good agreement with the measured overall rate constant but tend to overestimate the branching ratio defined as β = k1a/(k1a + k1b) at lower temperatures. Modest adjustments of the attractive potentials for the reaction yield values of k1a = 4.3 × 10–6(T/K)−2.191 exp(−229 K/T) cm3 molecule–1 s–1 and k1b = 1.5 × 10–12(T/K)0.032 exp(761 K/T) cm3 molecule–1 s–1, in good agreement with experiment, and we recommend these rate coefficients for use in modeling.