Ab initio chemical kinetics for the NH2 + HNOx reactions, part III: Kinetics and mechanism for NH2 + HONO2

✍ Scribed by Shucheng Xu; M. C. Lin

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 353 KB
• Volume: 42
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20463

✦ Synopsis

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)/6‐311+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…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 × 10^−3^ × T^−3.85^ exp(−96/T) cm^3^ molecule^−1^ s^−1^ at T = 200–550 K, 5.11 × 10^−23^ × 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~…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 × 10^−24^ × 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 × 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~. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 69–78, 2010