Ab Initio Chemical Kinetics for H plus NCN: Prediction of NCN Heat of Formation and Reaction Product Branching via Doublet and Quartet Surfaces

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (CCO)-C-3 + N-2 = (NCN)-N-3 + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN -> CH + N-2 (k(P1)), HCN + N-4 (k(QP1)), and HNC + N-4 (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N-2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by k(r) = 4.01 x 10(-15) T-0.90 exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.