EXPERIMENTAL-DETERMINATION OF THE RATE-CONSTANT FOR THE REACTION OF C2H3 WITH H-2 AND IMPLICATIONS FOR THE PARTITIONING OF HYDROCARBONS IN ATMOSPHERES OF THE OUTER PLANETS

The reaction between C2H3 and H-2, has been suggested to be potentiall y important in accounting for observational data on the abundance of l ow-molecular weight hydrocarbons in the atmospheres of the giant plane ts, especially the ratio of [C2H6]/[C2H2] in the jovian stratosphere. Previous values of the rate constant for this reaction have depended o n or are derived from either calculated estimates or very indirect mea surements in complex reaction systems, Further, reported values, both measured and calculated, for the rate constant k (C2H3 + H-2) at T = 2 98 K have ranged from 2.5 x 10(-17) to 9 x 10(-20) cm(3) molecule(-1) sec(-1). In this work the room temperature rate constant for the react ion of the vinyl radical with molecular hydrogen has been determined b y employing laser photolysis coupled to a kinetic-absorption spectrosc opic technique and separately via a gas chromatographic product analys is technique. In one set of experiments the vinyl radicals were genera ted through the lambda = 193 mm 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 comb ination product) absorption at lambda = 215 nm. By employing kinetic m odeling procedures a rate constant of (3 +/- 2) x 10(-20) cm(3) molecu le(-1) sec(-1) was derived. Independently, the lambda = 193 mm photoly sis of methyl vinyl ketone was used to generate nearly identical conce ntrations of methyl and vinyl radicals. Gas chromatographic analysis o f 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 x 10(-20) cm(3) molecule(-1) sec(-1). The planetary implications of the rate de terminations are briefly discussed. (C) 1995 Academic Press, Inc.