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

Abstract

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.

GRAPHICAL ABSTRACT

KEYWORDS:

• CH
• CH4
• SCTST
• HEAT
• flames

Acknowledgements

This work is dedicated to the occasion of Attila G. Császár’s 65th birthday. The senior author of this work (J.F.S.) has been both a friend and admirer of the work of Prof. Császár for many years. He is a true leader in bridging the gap between extremely high-level quantum chemistry and the prediction and analysis of experimental observables. Attila is a true leader in the world of applied theoretical chemical physics and his work has greatly benefitted that community. TLN and JFS conceived of the presented idea. TLN performed the computations. TLN and JFS wrote the manuscript. All authors discussed the results and contributed to the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available within the article and its supporting information. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Additional information

Funding

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award DE-SC0018164.