Ab Initio Chemical Kinetics for Singlet CH2 Reaction with N2 and the Related Decomposition of Diazomethane

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• 作者：Shucheng Xu ; M. C. Lin
• 刊名：Journal of Physical Chemistry A
• 出版年：2010
• 出版时间：April 22, 2010
• 年：2010
• 卷：114
• 期：15
• 页码：5195-5204
• 全文大小：320K
• 年卷期：v.114,no.15(April 22, 2010)
• ISSN：1520-5215

文摘

The kinetics and mechanism for the reaction of singlet state CH₂ with N₂ have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of the reactions has been calculated 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. By comparing the differences in the predicted heats of reaction with the available experimental values, we estimate the uncertainties in the calculated heats of reactions are ±1.4 kcal/mol. Rate constants for various product channels in the temperature range of 300−3000 K are predicted by the variational transition state and RRKM theories. The predicted total rate constants for ¹CH₂ + N₂ at 760 Torr Ar pressure can be represented by the expressions s-k_total = 9.67 × 10⁺⁷ × T ^−6.88 exp (−1345/T) cm³ molecule⁻¹ s⁻¹ at T = 300−2400 K and 3.15 × 10⁻²²⁹ × T ^+56.18 exp (128000/T) cm³ molecule⁻¹ s⁻¹ at T = 2400−3000 K. The branching ratios of the primary channels for ¹CH₂ + N₂ are predicted: k₁ for forming singlet s-CH₂N₂-a (diazomethane) accounts for 0.97−0.01, k₂ + k₄ for producing HCNN-a + H accounts for 0.00−0.69, k₃ for forming singlet s-CH₂N₂-b (3H-diazirine) accounts for 0.03−0.00, k₅ for producing HCN + NH accounts for 0.00−0.18, and k₆ for producing CNNH + H accounts for 0.00−0.11 in the temperature range of 300−3000 K. The rate constant predicted for the unimoclecular decomposition of diazomethane producing ¹CH₂ + N₂ agrees closely with experimental results. Because of the low stability of the two isomeric CH₂N₂ adducts and the high barriers for production of CN-containing products, the contribution of the CH₂ + N₂ reaction to NO formation becomes very small.