Ab initio chemical kinetics for singlet CH 2 reaction with N 2and the related decomposition of diazomethane

Shucheng Xu, Ming-Chang Lin

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The kinetics and mechanism for the reaction of singlet state CH 2 with N 2 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 1CH 2 + N 2 at 760 Torr Ar pressure can be represented by the expressions s-k2 = 9.67 x 10 +7 x T -6.88 exp (-1345/7) cm 3 molecule -1 s -1: at T = 300-2400 K and 3.15 x 10 -229 x T -56.18 exp (128 000/7) cm 3 molecule -1 s -1 at T = 2400-3000 K. The branching ratios of the primary channels for 1CH 2 + N 2 are predicted: k 1 for forming singlet s-CH 2N 2-a (diazomethane) accounts for 0.97-0.01, k 2 + k 4 for producing HCNN-a + H accounts for 0.00-0.69, k 3 for forming singlet s-CH 2N 2-b (3H-diazirine) accounts for 0.03-0.00, k 5 for producing HCN + NH accounts for 0.00-0.1.8, and k 6 for producing CNNH + H accounts for 0.00-0.11l in the temperature range of 300-3000 K. The rate constant predicted for the unimoclecular decomposition of diazomethane producing 1CH 2 + N 2 agrees closely with experimental results. Because of the low stability of the two isomeric CH 2N 2 adducts and the high barriers for production of CN-containing products, the contribution of the CH 2 + N 2 reaction to NO formation becomes very small.

Original languageEnglish
Pages (from-to)5195-5204
Number of pages10
JournalJournal of Physical Chemistry A
Issue number15
StatePublished - 22 Apr 2010

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