Branching ratio of the C 2 H 2 + O reaction at 290 K from kinetic modelling of relative methylene concentration versus time profiles in C 2 H 2 /O/H systems

In earlier work on the room temperature oxidation of C 2 H 2 by O atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel 1b of the C 2 H 2 + 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 C 2 H 2 /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 CH 2 concentration‐versus‐time profiles measured in a variety of C 2 H 2 /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 CH 2 ( 3 B 1 ) yield of the two‐channel C 2 H 2 + O reaction was determined to be k 1b / k 1 = 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 CH 2 profiles. The present result, which translates to an HCCO yield k 1a / k 1 = 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 [C 2 H 2 ], [O], and [H] profiles with an average accuracy of 5%, thus validating the consistency of the C 2 H 2 /O/H reaction model put forward here. © 1994 John Wiley & Sons, Inc.