Ethynyl Radical Hydrogen Abstraction Energetics and Kinetics Utilizing High-Level Theory.

The ethynyl radical, C 2 H, is found in a variety of different environments ranging from interstellar space and planetary atmospheres to playing an important role in the combustion of various alkynes under fuel-rich conditions. Hydrogen-atom abstraction reactions are common for the ethynyl radical in these contrasting environments. In this study, the C 2 H + HX → C 2 H 2 + X, where HX = HNCO, trans -HONO, cis -HONO, C 2 H 4 , and CH 3 OH, reactions have been investigated at rigorously high levels of theory, including CCSD(T)-F12a/cc-pVTZ-F12. For the stationary points thus located, much higher levels of theory have been used, with basis sets as large as aug-cc-pV5Z and methods up to CCSDT(Q), and core correlation was also included. These molecules were chosen because they can be found in either interstellar or combustion environments. Various additive energy corrections have been included to converge the relative enthalpies of the stationary points to subchemical accuracy (≤0.5 kcal mol -1 ). Barriers predicted here (2.19 kcal mol -1 for the HNCO reaction and 0.47 kcal mol -1 for C 2 H 4 ) are significantly lower than previous predictions. Reliable kinetics were acquired over a wide range of temperatures (50-5000 K), which may be useful for future experimental studies of these reactions.