Ab Initio Chemical Kinetics for Singlet CH2 Reaction with N-2 and the Related Decomposition of Diazomethane

Abstract

The kinetics and mechanism for the reaction of singlet state CH2 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 (CH2)-C-1 + N-2 at 760 Torr Ar pressure can be represented by the expressions s-k(total) = 9.67 x 10(+7) x T-6.88 exp (-1345/T) cm(3) molecule(-1) s(-1) at T = 300-2400 K and 3.15 x 10(-229) x T+56.18 exp (128 000/T) cm(3) molecule(-1) s(-1) at T = 2400-3000 K. The branching ratios of the primary channels for (CH2)-C-1 + N-2 are predicted: k(1) for forming singlet s-CH2N2-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-CH2N2-b (3H-diazirine) accounts for 0.03-0.00, k(5) for producing HCN + NH accounts for 0.00-0.18, and k(6) 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 (CH2)-C-1 + N-2 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 + N-2 reaction to NO formation becomes very small.