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

Abstract

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 (1)CH(2) + 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 (1)CH(2) + N(2) are predicted: k(1) for forming singlet s-CH(2)N(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(2)N(2)-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 (1)CH(2) + N(2) agrees closely with experimental results. Because of the low stability of the two isomeric CH(2)N(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.