Ab initio chemical kinetics for the NH₂ + HNOx Reactions, Part I: Kinetics and Mechanism for NH₂ + HNO

The kinetics and mechanism for the reaction of NH 2 with HNO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single‐point calculations at the CCSD(T)/6‐311+G(3 df , 2 p ) level based on geometries optimized at the CCSD/6‐311++G( d , p ) level. The major products of this reaction were found to be NH 3 + NO formed by H‐abstraction via a long‐lived H 2 N⋅⋅⋅HNO complex and the H 2 NN(H)O radical intermediate formed by association with 26.9 kcal/mol binding energy. The rate constants for formation of primary products in the temperature range of 300–3000 K were predicted by variational transition state or RRKM theories. The predicted total rate constants at the 760 Torr Ar pressure can be represented by k total = 3.83 × 10 −20 × T +2.47 exp(1450/T) at T = 300–600 K; 2.58 × 10 −22 × T +3.15 exp(1831/T) cm 3 molecule −1 s −1 at T = 600−3000 K. The branching ratios of major channels at 760 Torr Ar pressure are predicted: k 1 + k 3 + k 4 producing NH 3 + NO accounts for 0.59–0.90 at T = 300–3000 K peaking around 1000 K, k 2 accounts for 0.41–0.03 at T = 300–600 K decreasing with temperature, and k 5 accounts for 0.07–0.27 at T > 600 K increasing gradually with temperature. The NH 3 + NO formation rate constant was found to be a factor of 3–10 smaller than that of the isoelectronic reaction CH 3 + HNO producing CH 4 + NO, which has been shown to take place by barrierless H‐abstraction without involving a hydrogen‐bonding complex as in the NH 2 case.