Ab Initio Chemical Kinetics for the NH(2) + HNO(x) Reactions, Part I: Kinetics and Mechanism for NH(2) + HNO

Abstract

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(3df, 2p) 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 center dot center dot center dot 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 At pressure can be represented by k(total) = 3.83 x 10(-20) x T(+2.47)exp(1450/T) at T = 300-600 K; 2.58 x 10(-22) x 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 NH3 + 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. (C) 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 667-677, 2009