Conclusive determination of ethynyl radical hydrogen abstraction energetics and kinetics*

Abstract

The ethynyl radical (C${}_2$H) is ubiquitous, appearing in a variety of environments from interstellar space to oxy-acetylene flames. Under these diverse conditions, ethynyl has a high affinity to abstract hydrogen atoms from nearby molecules. In this study, ${\mathrm{C}}_2\mathrm{H}+\mathrm{HX}\to {\mathrm{C}}_2{\mathrm{H}}_2+\mathrm{X}$, $\mathrm{X}={\mathrm{CH}}_3$, NH${}_2$, OH, F, PH${}_2$, SH, Cl, C${}_2$H, CN, and NC reactions have been examined at a rigorously high level of theory. Various additive corrections have been appended to the coupled-cluster framework to converge the relative enthalpy of stationary points on the C${}_2$H + HX surfaces to subchemical accuracy (≤0.5 kcal mol${}^{-1}$). The barriers for the abstraction of a hydrogen from NH${}_3$, PH${}_3$, and H${}_2$S are submerged below the relative enthalpy of their reactant asymptotes by 2.75, 2.48, and 1.39 kcal mol${}^{-1}$, respectively. Abstractions from H${}_2$, CH${}_4$, H${}_2$O, and HCl are predicted to have moderate barriers to the abstraction of 2.64, 0.77, 3.82, and 2.19 kcal mol${}^{-1}$, respectively, suggesting these reactions will be significant to various systems at moderate to high temperatures. Accurate kinetics were obtained for the ${\mathrm{C}}_2\mathrm{H}+{\mathrm{H}}_2$, CH${}_4$, NH${}_3$, and H${}_2$O reactions over a wide range of temperatures with excellent agreement to present experimental rate constants.

GRAPHICAL ABSTRACT

Keywords:

• Ethynyl radical
• hydrogen abstraction
• focal point analysis
• kinetics

Acknowledgments

The authors are grateful for helpful comments and suggestions by Dr. Sarah N. Elliot.

Disclosure statement

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

Additional information

Funding

The authors acknowledge support from the US Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Grant No. DE-SC0018412.