Kinetics of the Reactions of C2H with C2H2, H2, and D2

Article abstract of DOI:10.1021/j100193a043

Kinetics of the reactions C2H + C2H2 --> C4H2 + H (1), C2H + H2 --> C2H2 + H (2), and C2H + D2 --> C2HD + D (3) have been studied at T = 293 K with two time-resolved diagnostics, laser-induced fluorescence (LIF) detection of H (or D) atoms at the Lyman-α wavelength and mass spectrometric detection of C4H2.The C2H radical was prepared by the ArF (193 nm) laser photolysis of C2H2.Measurements of the yield of H (or D) atom indicated that the C2H radicals produced by the photolysis were converted effectively to H (or D) atoms; i.e. formation of the stable intermediatessuch as C4H3, C2H3, or C2HD2 was negligible under the present experimental conditions (p = 30 Torr).Rate constants of k₁ = (1.6 +/- 0.1) * 10^-10, k₂ = (7.1 +/- 1.1) * 10^-13, and k₃ = (2.0 +/- 0.3) * 10^-13 cm³ molecule^-1 s^-1 were derived from the pseudo-first-order rise rates of the H (or D) atom.Rate constant of k₁ = (1.4 +/- 0.3) * 10^-10 cm³ molecule^-1 s^-1 was also obtained at p = 5 Torr by measuring the appearance rate of C4H2.The ratios of the rate constants k₂/k₁ = (3.4 +/- 0.2) * 10^-3 and k₃/k₁ = (1.4 +/- 0.5) * 10^-3 were deduced by measuring the dependence of the C4H2 production on the partial pressure of added H2 or D2.Transition state theory calculations on the basis of ab initio transition-state properties demonstrated the importance of the tunnel effect for reaction 2 at room temperature.