Theoretical Study of the C(2)H(5) + HO(2) Reaction: Mechanism and Kinetics

The mechanism and kinetics for the reaction of the HO(2) radical with the ethyl (C(2)H(5)) 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 HO(2) radical can attack C(2)H(5) via a barrierless addition mechanism to form the energy-rich intermediate IM1 C(2)H(5)OOH (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 P(1) C(2)H(5)O + 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 P(1) 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 P(16) C(2)H(6) + (3)O(2) 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 C(2)H(5) + HO(2) reaction under atmospheric and practical combustion conditions.