Theoretical Study of the C₂H₅ + HO₂ Reaction: Mechanism and Kinetics

The mechanism and kinetics for the reaction of the HO₂ radical with the ethyl (C₂H₅) radical have been investigated theoretically. The electronic structure information of the potential energy surface (PES) is obtained at the MP2/6-311++G(d,p) level of theory, and the single-point energies are refined by the CCSD(T)/6-311+G(3df,2p) level of theory. The kinetics of the reaction with multiple channels have been studied by applying variational transition-state theory (VTST) and Rice⁻Ramsperger⁻Kassel⁻Marcus (RRKM) theory over wide temperature and pressure ranges (T = 220⁻3000 K; P = 1 × 10^-4⁻100 bar). The calculated results show that the HO₂ radical can attack C₂H₅ via a barrierless addition mechanism to form the energy-rich intermediate IM1 C₂H₅OOH (68.7 kcal/mol) on the singlet PES. The collisional stabilization intermediate IM1 is the predominant product of the reaction at high pressures and low temperatures, while the bimolecular product P₁ C₂H₅O + OH becomes the primary product at lower pressures or higher temperatures. At the experimentally measured temperature 293 K and in the whole pressure range, the reaction yields P₁ as major product, which is in good agreement with experiment results, and the branching ratios are predicted to change from 0.96 at 1 × 10^-4 bar to 0.66 at 100 bar. Moreover, the direct H-abstraction product P₁₆ C₂H₆ + ³O₂ on the triplet PES is the secondary feasible product with a yield of 0.04 at the collisional limit of 293 K. The present results will be useful to gain deeper insight into the understanding of the kinetics of the C₂H₅ + HO₂ reaction under atmospheric and practical combustion conditions.