Kinetics of CH + N₂ Revisited with Multireference Methods

Abstract

The potential energy surface for the CH + N2 reaction was reexamined with multireference ab initio electronic structure methods employing basis sets up to aug-cc-pvqz. Comparisons with related CCSD(T) calculations were also made. The multireference ab initio calculations indicate significant shortcomings in single reference based methods for two key rate-limiting transition states. Transition state theory calculations incorporating the revised best estimates for the transition state properties provide order of magnitude changes in the predicted rate coefficient in the temperature range of importance to the mechanism for prompt NO formation. At higher temperatures, two distinct pathways make a significant contribution to the kinetics. A key part of the transition state analysis involves a variable reaction coordinate transition state theory treatment for the formation of H + NCN from HNCN. The present predictions for the rate coefficients resolve the discrepancy between prior theory and very recent experimental measurements.

Uncertainty analysis of NO production during methane combustion

“…The CH measurements established NCN and H as the primary products of the CH + N 2 reaction. These results were confirmed by Harding et al [42], who investigated the potential energy surface for the CH + N 2 reaction with multireference ab initio electronic structure methods. In the cases of both steps ((R2a) and (R2b)), the intermediate formed (HCN or NCN) is converted to NO through several steps.…”

Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH4–O2–N2 and C2H2–O2–N2 flames

“…It has been established that NCN and H were the primary products, with a branching ratio of 1. The uncertainty of the rate constant k 1 was estimated to be around ±35% at 2100 K and ±25% at 3350 K. In addition, the accuracy of the expression given from theoretical computations by Harding et al [10] was estimated within a factor of 2 in the temperature range 1000-2000 K. With respect to these uncertainties, the rate constant expression has been adjusted with respect to the experimental peak values of both NO and NCN, as specified in Table 3. Fig.…”

Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: The importance of internal hydrogen bonding at the transition state

“…[24][25][26][27][28][29][30][31][32] ͒ Recently, calculations on the reactions of OH with propene were reported by Izsák et al, 33 who employed CASPT2 geometry optimizations. Harding et al 34,35 have obtained good results with CASPT2-optimized transition states.…”