Kinetics of the reaction between oxygen atoms and ethyl radicals

Abstract

The kinetics of the O + C₂H₅ reaction have been investigated experimentally in a heatable tubular reactor coupled to a photoionization mass spectrometer. The reactants were generated homogeneously in the reactor by the simultaneous in situ photolysis of SO₂ and diethyl ketone. The rate constant was determined from time-resolved measurements of C₂H₅ decay profiles under conditions of [O] in express [O]/[C₂H₅]₀ > 20. The value of the O + C₂H₅ rate constant [(2.2 ± 0.4)×10^–10 cm³ molecule^–1 s^–1] was independent of temperature (295–600K) and density [(3–12)×10¹⁶ molecule cm^–3]. The chemical branching was also characterized both experimentally and theoretically. In the experimental study, the absolute yields of products from three reactive routes were determined and found also to be independent of temperature (298–450K) and density (same range). The branching fractions are 0.32 ± 0.07 for CH₂O + CH₃, 0.40 ± 0.04 for CH₃CHO + H and 0.23 ± 0.8 for C₂H₄+ OH. In the theoretical study, the master equation describing the many possible intramolecular dynamical processes of the excited C₂H₅O formed in the initial step of this reaction was solved. RRKM theory was used to obtain the transition probabilities for each step. Potential–energy surface information needed to obtain the transition probabilities was obtained using BAC–MP4 electronic structure calculations. Theoretical branching fractions are in good agreement with those measured for the two major routes. Far less C₂H₄+ OH is predicted than observed, suggesting that these products are primarily produced by another process, viz. direct metathesis.