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

Abstract

The kinetics and mechanism for the reaction of NH(2) with HONO(2) 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)/6311 +G(3df, 2p) level based on geometries optimized at the B3LYP/6-311 +G(3df, 2p) level. The reaction producing the primary products, NH(3) + NO(3), takes place via a precursor complex, H(2)N center dot center dot center dot HONO(2) with an 8.4-kcal/mol binding energy. The rate constants for major product channels in the temperature range 200-3000 K are predicted by variational transition state or variational Rice-Ramsperger-Kassel-Marcus theory. The results show that the reaction has a noticeable pressure dependence at T < 900 K. The total rate constants at 760 Torr Ar-pressure can be represented by k(total) = 1.71 x 10(-3) x T(-3.85) exp(-96/T) cm(3) molecule(-1) s(-1) at T = 200-550 K, 5.11 X 10(-23) x T(+3.22) exp(70/T) cm(3) molecule(-1) s(-1) at T = 550-3000 K. The branching ratios of primary channels at 760 Torr Ar-pressure are predicted: k(1) producing NH(3) + NO(3) accounts for 1,00-0,99 in the temperature range of 200-3000 K and k(2) + k(3) producing H(2)NO + HONO accounts for less than 0.01 when temperature is more than 2600 K. The reverse reaction, NH(3) + NO(3) -> NH(2) + HONO(2) shows relatively weak pressure dependence at P < 100 Torr and T < 600 K due to its precursor complex, NH(3)center dot center dot center dot O(3)N with a lower binding energy of 1.8 kcal/mol, The predicted rate constants can be represented by k(-1) = 6.70 x 10(-24) x T(+3.58) exp(-850/T) cm(3) molecule(-1) s(-1) at T = 200-3000 K and 760 Torr N(2) pressure, where the predicted rate at T = 298 K, 2.8 x 10(-16) cm(3) molecule(-1) s(-1) is in good agreement with the experimental data. The NH(3) + NO(3) formation rate constant was found to be a factor of 4 smaller than that of the reaction OH + HONO(2) producing the H(2)O + NO(3) because of the lower barrier for the transition state for the OH + HONO(2) (C) 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 69-78, 2010