Ab Initio Chemical Kinetics for the NH2 + HNO_x Reactions, Part I: Kinetics and Mechanism for NH2 + HNO

摘要

The kinetics and mechanism for the reaction of NH2 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)/6-31 1+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 NH3 + NO formed by H-abstraction via a long-lived H2N···HNO complex and the H2NN(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 x 10~(-20)xT~(+2.47)exp(1450/T) at T = 300-600 K; 2.58 x 10~(-22) xT~(+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: k1 + k3 + k4 producing NH3 + NO accounts for 0.59-0.90 at T = 300-3000 K peaking around 1000 K, k2 accounts for 0.41-0.03 at T = 300-600 K decreasing with temperature, and k5 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 CH3 + HNO producing CH4 + NO, which has been shown to take place by barrierless H-abstraction without involving a hydrogen-bonding complex as in the NH2 case.