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 <i>k</i>_1,exp(<i>T</i>) = 9.5 × 10^–7(<i>T</i>/K)^−2.05 exp­(−404 K/<i>T</i>) 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 <i>k</i>_1,theory(<i>T</i>) = 7.5 × 10^–12(<i>T</i>/K)^−0.172 exp­(687 K/<i>T</i>) 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 = <i>k</i>_1a/(<i>k</i>_1a + <i>k</i>_1b) at lower temperatures. Modest adjustments of the attractive potentials for the reaction yield values of <i>k</i>_1a = 4.3 × 10^–6(<i>T</i>/K)^−2.191 exp­(−229 K/<i>T</i>) cm³ molecule^–1 s^–1 and <i>k</i>_1b = 1.5 × 10^–12(<i>T</i>/K)^0.032 exp­(761 K/<i>T</i>) cm³ molecule^–1 s^–1, in good agreement with experiment, and we recommend these rate coefficients for use in modeling