KINETICS OF THE REACTIONS OF C2H WITH C2H2, H2, AND D2

Kinetics of the reactions C2H + C2H2 --> C4H2 + H (1), C2H + H-2 --> 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-alpha 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 intermediates such as C4H3, C2H3, or C2HD2 was negligible under the present experimental conditions (p = 30 Torr). Rate constants of k1 = (1.6 +/- 0.1) x 10(-10), k2 = (7.1 +/- 1.1) X 10(-13), and k3 = (2.0 +/- 0.3) x 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) x 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) X 10(-3) and k3/k1 = (1.4 +/- 0.5) X 10(-3) were deduced by measuring the dependence of the C4H2 production on the partial pressure of added H-2 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.