Rate Constant and Branching Ratio for the Reactions of the Ethyl Peroxy Radical with Itself and with the Ethoxy Radical

Abstract

The self-reaction of the ethyl peroxy radical (C2H5O2) (1) has been studied using laser photolysis coupled to a selective time-resolved detection of two different radicals by continuous-wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared range: C2H5O2 was detected in the Ã-X̃ electronic transition at 7596 cm–1 and HO2 was detected in the 2ν1 vibrational overtone transition at 6638.2 cm–1. Radicals were generated from pulsed 351 nm photolysis of the C2H6/Cl2 mixture in the presence of O2. The reaction can proceed via a radical propagation channel, leading to two C2H5O radicals (1a) or to stable products (1b/1c). Because C2H5O radicals react subsequently with O2, leading to HO2, which in turn reacts rapidly with C2H5O2, knowledge of the branching ratio is indispensable for retrieving the rate constant. A strong disagreement exists about the rate constant and the branching ratio of (1) between the recommendation of the IUPAC (Atkinson et al., Atmos. Chem. Phys. 2006, 3625) or JPL (Publication 19-5, 2020) and a recent, more direct measurement (Noell et al., J. Phys. Chem. A 2010, 6983). The rate constant for the self-reaction has been found here to be k1 = (1.0 ± 0.2) × 10–13 cm3 s–1 with the branching fraction of the C2H5O radical channel being α = (0.31 ± 0.06), being in contradiction with the recommendations, but confirming the most recent measurement and indicating that the current recommendation for this reaction should be revised. The reaction of C2H5O2 with the ethoxy radical (C2H5O) can also be involved in the reaction mechanism, and its rate constant is also extracted from modeling. It is found to be k9 = (7 ± 1.5) × 10–12 cm3 s–1, two times slower than the only available measurement from Noell et al. J. Phys. Chem. A 2010, 6983.