The reaction of methylidyne with methane: role of the quartet electronic state

The reaction of methylidyne (CH) with methane, which plays an important role in combustion and planetary atmospheres, is characterised with high accuracy coupled-cluster calculations. The reaction on the quartet state potential energy surface (PES) is predicted to lead to direct H-abstraction with a small barrier of 2.2 kcal mol−1, yielding triplet CH2 and CH3. The reaction on the doublet state PES is barrierless, involving an insertion/elimination mechanism that leads to the vibrationally excited (hot) intermediate C2H5, which promptly dissociates to H + C2H4. The results obtained from a statistical chemical kinetics analysis show that the quartet channel makes a negligible contribution to the overall kinetics below 500 K, but becomes important at the higher temperatures characteristic of hydrocarbon flames. Reaction rate coefficients calculated from first principles are in good agreement (within 20%) with experimental values where they are available (200–800 K). For the higher temperatures of combustion environments (where experimental data are lacking), the present study provides high-level results for kinetics modelling purposes.