Absolute Rate Constant and Product Branching Ratios for the Reaction between H and C2H3 at T=213 and 298 K

Article abstract of DOI:10.1021/j100047a018

The discharge-flow kinetic technique coupled to mass-spectrometric detection has been used to determine the variable-temperature dependence of the rate constant and product branching ratios for the reaction between H and C2H3 at 1 Torr nominal pressure (He).Atomic hydrogen was produced from the reaction between F(²P) and H2 while the vinyl radical produced simultaneously from the reaction between F(²P) and ethylene, which gives both C2H3 and H.The reaction was studied at T=213 and 298 K by monitoring the decay of C2H3 in the presence of a large excess of H.The rate constants were determined to be k(H+C2H3)(293 K)=(1.1 +/-0.3)*10^-10 and k(H+C2H3)(213 K)=(1.0 +/-0.3)*10^-10 both in the units cm³ molecule^-1 s^-1; the quoted uncertainty represents total errors.The activation energy for the reaction between H and C2H3 is therefore near zero over the temperature range studied.Further, the fractional product yields for the channels H+C2D3 -> C2D3H (a) and H+C2D3 -> C2D2+HD (b) were determined by quantitatively measuring the yiels of both C2D3H and HD independently.The derived fractional product yields were Γ_a(298 K)=0.33 +/-0.13, Γ_b(298 K)=0.67 +/-0.18, Γ_a(213 K)=0.24 +/-0.09, and Γ_b(213)=0.76 +/-0.16, where the quoted uncertainty represents total errors.Quantum RRK (QRRK) calculations have been undertaken to investigate the relationship between the observed kinetics, products, and possible mechanisms.With the available data and the QRRK calculations, a mechanism of the form H+C2H3+M <-> ^* -> C2H4+M (a) and H+C2H3 -> H2+C2H2 (b) is shown to be most likely.Further, Troe calculations have been undertaken in order to suggest values for the limiting low-pressure rate coefficients.A brief comparison is made between the results of the Troe and QRRK analyses.The implications for the conversion of C2H2 to C2H6 in the relatively low temperature conditions of planetary athmospheres are briefly discussed.