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

Abstract

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, (3)CCO + N2 = (3)NCN + 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 + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (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 + N2 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 kr = 4.01 × 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.

Modeling nitrogen chemistry in combustion

“…Knowledge of the subsequent reactions of the NCN radical has improved con- siderably in recent years, owing to a large degree to the shock tube work of Friedrichs and coworkers who determined rate constants for several of the NCN reactions, i.e., NCN + M (R7) [83], NCN + H [78], NCN + O (R9) [83], NCN + O 2 (R13) [85], and NCN + NO (R14) [86]. In addition, Lin and coworkers have studied theoretically a number of reactions, including NCN + H [92], NCN + O [84], NCN + OH [93] and NCN + O 2 [94]. Very recently, Klippenstein et al [82] reported a theoretical study of NCN + H and NCN + OH.…”

The rate constant of the reaction NCN + H2 and its role in NCN and NO modeling in low pressure CH4/O2/N2-flames

“…Whatever channel (d-f) of reaction (2) was assumed, simulated NO profiles are identical to each other. However, considering channel (2d), implementation of reactions (12)(13)(14)(15)(16)(17)(18)(19) could be removed since HCN and NH species are already declared in GDF. 13 In that case, simulated NO profiles would be identical to those obtained with the original GDF mechanism.…”

“…They reported RRKM and TST studies for the most important bimolecular NCN reactions NCN + O 2 , 16 O, 17 OH 18 and H 19 as well as for the NCN forming reaction CH + N 2 . 6,19 From the experimental point of view, most direct rate constant measurements of NCN reactions have been accomplished over the past few years in the shock tube laboratory of the authors (N.F. and G.F.).…”

Collisional Relaxation of NCN: An Experimental Study with Laser-Induced Fluorescence

“…In combustion chemistry, the kinetics of the cyanonitrene radical, NCN, has recently attracted attention [4][5][6][7][8][9][10][11] because NCN is considered an important intermediate in the formation of so-called prompt NO in hydrocarbon flames under fuel-rich conditions [12][13][14][15][16][17][18][19][20]. Cyanonitrene is a linear diradical with a 3 − g electronic ground state [21,22] and was shown to be formed under combustion conditions in the spin-allowed reaction CH(…”