Ab initio chemical kinetics for the NH2 + HNOx reactions, part II: Kinetics and mechanism for NH2 + HONO

✍ Scribed by Shucheng Xu; M. C. Lin

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 405 KB
• Volume: 41
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20445

✦ Synopsis

Abstract

The kinetics and mechanism for the reaction of NH~2~ 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, 2__p__) level based on geometries optimized at the CCSD/6‐311++G(d, p) level. The reaction producing the primary products, NH~3~ + NO~2~, takes place via precomplexes, H~2~N⋅⋅⋅c‐HONO or H~2~N⋅⋅⋅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 × 10^−20^ × T^2.34^ exp(1612/T) cm^3^ molecule^−1^ s^−1^ at T = 300–650 K and 8.04 × 10^−22^ × 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 NH~3~ + NO~2~ accounts for 1.00–0.98 in the temperature range 300–3000 K and k~2~ producing OH + H~2~NNO accounts for 0.02 at T > 2500 K. The predicted rate constant for the reverse reaction, NH~3~ + NO~2~ → NH~2~ + HONO represented by 8.00 × 10^−26^ × T^4.25^ exp(−11,560/T) cm^3^ molecule^−1^ s^−1^, is in good agreement with the experimental data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 678–688, 2009