Temperature Dependence of the Rate Constants for the Reactions of C2H with C2H2, H2, and D2

Article abstract of DOI:10.1021/j100203a048

Reactions of ethynyl (C2H) radical with C2H2, H2, and D2 were studied over the temperature range 298-438 K by time-resolved mass spectrometry.The rate of the reaction with C2H2 was followed by measuring the appearance rate of C4H2.The rates of the reactions with H2 and D2 were deduced by measuring the dependence of C4H2 production (arising from the reaction C2H with the C2H2 precursor) on the partial pressure of added H2 or D2.The rate constants for the reaction C2H + C2H2 -> C4H2 + H were also measured following reflected shock waves by monitoring H atom resonant absorption at 121.6 nm.In both experiments, the C2H radical was generated by ArF (193-nm) laser photolysis of C2H2.A rate constant of (1.5 +/- 0.3) * 10^-10 cm³ molecule^-1 s^-1 was obtained for reaction 1 without any temperature dependence at T = 298-2177 K.The results for the reactions C2H + H2 -> C2H2 + H and C2H + D2 -> C2HD + D could be represented by the Arrhenius expressions, k₂ = (1.8 +/- 1.0) * 10^-11 exp(-(1090+/-299)/T) and k₃ = (1.4+/-0.8) * 10^-11 exp(-(1377+/-301)/T) cm³ molecule^-1 s^-1, over the range of T = 298-438 K.The classical barrier height for reactions 2 and 3 was estimated to be 2 kcal/mol on the basis of conventional transition-state theory.The isotope effects on reactions 2 and 3 calculated with Wigner tunneling correction were in good agreement with the present results.