Ab initio Chemical Kinetics for the NH2 + HNOx Reactions, Part II: Kinetics and Mechanism for NH2 + HONO

Abstract

The kinetics and mechanism for the reaction of NH2 with HONO 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 reaction producing the primary products, NH3 + NO2, takes place via precomplexes, H2N center dot center dot center dot c-HONO or H2N center dot center dot center dot t-HONO with binding energies, 5.0 or 5.9 kcal/mol, respectively. The rate constants for the major reaction channels in the temperature range of 300-3000 K are predicted by variational transition state theory or Rice-Ramsperger-Kassel-Marcus theory depending on the mechanism involved. The total rate constant can be represented by k(total) = 1.69 x 10(-20) x T-2.34 exp(1612/T) cm(3) molecule(-1) s(-1) at T =300-650 K and 8.04 x 10(-22) x T-3.36 exp(2303/T) cm(3) molecule(-1) s(-1) at T =650-3000 K, The branching ratios of the major channels are predicted: k(1) + k(3) producing NH3 + NO2 accounts for 1.00-0.98 in the temperature range 300-3000 K and k(2) producing OH + H2NNO accounts for 0.02 at T > 2500 K. The predicted rate constant for the reverse reaction, NH3 + NO2 -> NH2 + HONO represented by 8.00 x 10(-26) x T-4.25 exp(-11,560/T) cm(3) molecule(-1) s(-1), is in good agreement with the experimental data. (C) 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 678-688, 2009