Conclusive determination of ethynyl radical hydrogen abstraction energetics and kinetics*

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

The ethynyl radical, C2H, 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 C2H + HX → C2H2 + X, where HX = HNCO, trans-HONO, cis-HONO, C2H4, and CH3OH, 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 C2H4) 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.

Energetics and kinetics of various cyano radical hydrogen abstractions

The cyano radical (CN) is an abundant, open-shell molecule found in a variety of environments, including the atmosphere, the interstellar medium and combustion processes. In these environments, it often reacts with small, closed-shell molecules via hydrogen abstraction. Both carbon and nitrogen atoms of the cyano radical are reactive sites, however the carbon is more reactive with reaction barrier heights generally between 2-15 kcal mol^-1 lower than those of the analogous nitrogen. The CN + HX → HCN/HNC + X, with X = H, CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl, C₂H, CN reactions have been studied at a high-level of theory, including CCSD(T)-F12a. Furthermore, kinetics were obtained over the 100-1000 K temperature range, showing excellent agreement with those rate constants that have been determined experimentally.