ABSOLUTE RATE-CONSTANT AND PRODUCT BRANCHING RATIOS FOR THE REACTION BETWEEN H AND C2H3 AT T=213 AND 298 K

The discharge-flow kinetic technique coupled to mass-spectrometric det ection has been used to determine the variable-temperature dependence of the rate constant and product branching ratios for the reaction bet ween H and C2H3 at 1 Torr nominal pressure (He). Atomic hydrogen was p roduced from the reaction between F(P-2) and H-2 while the vinyl radic al was produced simultaneously from the reaction between F(P-2) and et hylene, which gives both C2H3 and H. The reaction was studied at T = 2 13 and 298 K by monitoring the decay of C2H3 in the presence of a larg e excess of H. The rate constants were determined to be k(H + C2H3)(29 8 K) = (1.1 +/- 0.3) x 10(-10) and k(H+C2H3)(213 K) = (1.0 +/- 0.3) x 10(-10) both in the units cm(3) molecule(-1) s(-1); the quoted uncerta inty represents total errors. The activation energy for the reaction b etween H and C2H3 is therefore near zero over the temperature range st udied. Further, the fractional product yields for the channels H + C2D 3 --> C2D3H (a) and H + C2D3 --> C2D2 + HD (b) were determined by quan titatively measuring the yields of both C2D3H and HD independently. Th e derived fractional product yields were Gamma(a)(298 K) = 0.33 +/- 0. 13, Gamma(b)(298 K) = 0.67 +/- 0.18, Gamma(a)(213 K) = 0.24 +/- 0.09, and Gamma(b)(213 K) = 0.76 +/- 0.16, where the quoted uncertainty repr esents total errors. Quantum RRK (QRRK) calculations have been underta ken to investigate the relationship between the observed kinetics, pro ducts, and possible mechanisms. With the available data and the QRRK c alculations, a mechanism of the form H + C2H3 + M <-> [H-C2H3] --> C2 H4 + M (a) and H + C2H3 --> H-2 + C2H2 (b) is shown to be most likely. Further, Tree calculations have been undertaken in order to suggest v alues for the limiting low-pressure rate coefficients. A brief compari son is made between the results of the Tree and QRRK analyses. The imp lications for the conversion of C2H2 to C2H6 in the relatively low tem perature conditions of planetary atmospheres are briefly discussed.