The spin-conserved reaction CH + n₂ → H + NCN: A major pathway to prompt no studied by quantum/statistical theory calculations and kinetic modeling of rate constant

A new spin-conserved path for the CH(₂Π) + N₂ reaction at temperatures relevant to prompt NO formation has been theoretically investigated by means of ab initio MO calculations at the G2M level of theory. The result of the calculation reveals that the CH(²Π) + N₂ reaction takes place primarily via the ground electronic doublet potential energy surface, producing H- NCN instead of the commonly assumed, spin-forbidden HCN + N(⁴S) products. The overall rate constant for NCN production has been computed by a multichannel canonical variational Rice-Ramsperger-Kassel-Marcus theory calculation for the temperature range 1500-4000 K at 0.5-2 atm pressure: κ₃ = 2.22 × 10⁷ T^1.48 exp (-11760/T) cm³/ (mol • s). The theoretically predicted rate constant was found to be in good agreement with high-temperature shock tube data kinetically modeled with the new mechanism that includes NCN reactions. In addition, κ₃ was also found to be consistent with the apparent rate constants previously modeled for prompt NO formation in several flame studies.