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

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.