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

The self-reaction of the ethyl peroxy radical (C 2 H 5 O 2 ) (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: C 2 H 5 O 2 was detected in the Ã-X ̃ electronic transition at 7596 cm –1 and HO 2 was detected in the 2ν 1 vibrational overtone transition at 6638.2 cm –1 . Radicals were generated from pulsed 351 nm photolysis of the C 2 H 6 /Cl 2 mixture in the presence of O 2 . The reaction can proceed via a radical propagation channel, leading to two C 2 H 5 O radicals (1a) or to stable products (1b/1c). Because C 2 H 5 O radicals react subsequently with O 2, leading to HO 2, which in turn reacts rapidly with C 2 H 5 O 2, 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 k 1 = (1.0 ± 0.2) × 10 –13 cm 3 s –1 with the branching fraction of the C 2 H 5 O 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 C 2 H 5 O 2 with the ethoxy radical (C 2 H 5 O) can also be involved in the reaction mechanism, and its rate constant is also extracted from modeling. It is found to be k 9 = (7 ± 1.5) × 10 –12 cm 3 s –1, two times slower than the only available measurement from Noell et al. J. Phys. Chem. A 2010, 6983.