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

The rate constant and product branching for the self reaction of C 2 H 3 has been measured using the discharge−flow kinetic technique coupled to mass spectrometric detection at T = 298 K and 1 Torr nominal pressure (He). C 2 H 3 is produced by the reaction of F with C 2 H 4, which also forms C 2 H 3 F + H. In addition to the C 2 H 3 self reaction, C 2 H 3 also decays by reaction with H and by wall loss processes. The result obtained by parameter fitting the C 2 H 3 decay curves was k (C 2 H 3 + C 2 H 3 ) = (1.41 ± 0.60) × 10 -10 cm 3 molecule -1 s -1, where k is defined by d[C 2 H 3 ]/d t = 2 k [C 2 H 3 ] 2 . Results from the product studies showed that the recombination product 1,3-butadiene was not observed at 1 Torr and that the ratio [C 2 H 2 ] product formed/[C 2 H 3 ] 0 was 0.65 ± 0.14 for the combined C 2 H 3 + C 2 H 3 and C 2 H 3 + H reactions. Both observations are consistent with C 2 H 2 + C 2 H 4 being the exclusive C 2 H 3 + C 2 H 3 products, since the maximum yield of C 2 H 2 from the combined C 2 H 3 + C 2 H 3 and C 2 H 3 + H reactions is 0.59. The experimental observations that k 1 is independent of pressure and that no 1,3-butadiene (product of C 2 H 3 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 C 2 H 2 and C 2 H 4 . 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.