Branching ratio of the C2H2 + O reaction at 290 K from kinetic modelling of relative methylene concentration versus time profiles in C2H2/O/H systems

Article abstract of DOI:10.1002/kin.550260902

In earlier work on the room temperature oxidation of C₂H₂ by O atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel 1b of the C₂H₂ + O reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel 1a. Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C₂H₂/O systems depend strongly on the k_1a/k_1b branching ratio. Along that line, the important parameter k_1a/k_1b was determined from relative CH₂ concentration-versus-time profiles measured in a variety of C₂H₂/O/H systems using Discharge Flow-Molecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the CH₂(³B₁) yield of the two-channel C₂H₂ + O reaction was determined to be k_1b/k₁ = 0.17 ± 0.08. The indicated 2σ error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH₂ profiles. The present result, which translates to an HCCO yield k_1a/k₁ = 0.83 ± 0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [C₂H₂], [O], and [H] profiles with an average accuracy of 5%, thus validating the consistency of the C₂H₂/O/H reaction model put forward here.