Rate Constant and Product Branching for the Vinyl Radical Self Reaction at T = 298 K

Abstract

The rate constant and product branching for the self reaction of C2H3 has been measured using the discharge−flow kinetic technique coupled to mass spectrometric detection at T = 298 K and 1 Torr nominal pressure (He). C2H3 is produced by the reaction of F with C2H4, which also forms C2H3F + H. In addition to the C2H3 self reaction, C2H3 also decays by reaction with H and by wall loss processes. The result obtained by parameter fitting the C2H3 decay curves was k(C2H3 + C2H3) = (1.41 ± 0.60) × 10-10 cm3 molecule-1 s-1, where k is defined by d[C2H3]/dt = 2k[C2H3]2. Results from the product studies showed that the recombination product 1,3-butadiene was not observed at 1 Torr and that the ratio [C2H2] product formed/[C2H3]0 was 0.65 ± 0.14 for the combined C2H3 + C2H3 and C2H3 + H reactions. Both observations are consistent with C2H2 + C2H4 being the exclusive C2H3 + C2H3 products, since the maximum yield of C2H2 from the combined C2H3 + C2H3 and C2H3 + H reactions is 0.59. The experimental observations that k1 is independent of pressure and that no 1,3-butadiene (product of C2H3 combination) is observed at 1 Torr pressure requires a mechanism in which the chemically activated 1,3-butadiene undergoes a unimolecular reaction. It is postulated that the 1,3-butadiene first isomerizes to cyclobutene, which then unimolecularly decomposes to C2H2 and C2H4. Although the former reaction is well documented, the latter reaction has not been previously reported. RRKM calculations predict a pressure dependence similar to what is experimentally observed.