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 k(1,exp)(T) = 9.5 X 10(-7)(T/K)(-2.05) exp(-404 KIT) cm(3) 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 k(1, theory)(T) = 7.5 X 10(-12)(T/K)(-0.172) exp(687 KIT) cm(3) 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 beta = k(1a)/(k(1a) + k(1b)) at lower temperatures. Modest adjustments of the attractive potentials for the reaction yield values of k(1a) = 4.3 x 10(-6)(T/K)(-2.191) exp(-229 KIT) cm(3) molecule(-1) s(-1) and k(1b) = 1.5 X 10(-12)(T/K)(0.032) exp(761 KIT) cm(3) molecule(-1) s(-1), in good agreement with experiment, and we recommend these rate coefficients for use in modeling.