Experimental Determination of the Rate Constant for the Reaction of C 2H 3 with H 2 and Implications for the Partitioning of Hydrocarbons in Atmospheres of the Outer Planets

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The reaction between C 2H 3 and H 2 has been suggested to be potentially important in accounting for observational data on the abundance of low-molecular weight hydrocarbons in the atmospheres of the giant planets, especially the ratio of [C 2H 6]/[C 2H 2] in the jovian stratosphere. Previous values of the rate constant for this reaction have depended on or are derived from either calculated estimates or very indirect measurements in complex reaction systems. Further, reported values, both measured and calculated, for the rate constant k (C 2H 3 + H 2) at T = 298 K have ranged from 2.5 × 10 -17 to 9 × 10 -20 cm 3 molecule -1 sec -1. In this work the room temperature rate constant for the reaction of the vinyl radical with molecular hydrogen has been determined by employing laser photolysis coupled to a kinetic-absorption spectroscopic technique and separately via a gas chromatographic product analysis technique. In one set of experiments the vinyl radicals were generated through the λ = 193 nm photolysis of divinyl mercury using an ArF excimer laser. The time history of vinyl radicals in the presence of hydrogen was monitored using the 1,3-butadiene (vinyl radical combination product) absorption at λ = 215 nm. By employing kinetic modeling procedures a rate constant of (3 ± 2) × 10 -20 cm 3 molecule -1 sec -1 was derived. Independently, the λ = 193 nm photolysis of methyl vinyl ketone was used to generate nearly identical concentrations of methyl and vinyl radicals. Gas chromatographic analysis of the reaction products of the methylvinyl mixed radical system in the presence of H 2 result in a rate constant of the order of 1 × 10 -20 cm 3 molecule -1 sec -1. The planetary implications of the rate determinations are briefly discussed.