TEMPERATURE-DEPENDENCE OF THE RATE CONSTANTS FOR THE REACTIONS OF C2H WITH C2H2, H2, AND D2

Reactions of the ethynyl (C2H) radical with C2H2, H-2, and D2 were studied over the temperature range 298-438 K by time-resolved mass spectrometry. The mte of the reaction with C2H2 Was followed by measuring the appearance rate of C4H2. The rates of the reactions with H-2 and D2 Were deduced by measuring the dependence of C4H2 production (arising from the reaCtion Of C2H with the C2H2 precursor) on the partial pressure of added H-2 or D2. The mte 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) x 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 + H-2 --> C2H2 + H and C2H + D2 --> C2HD + D could be represented by the Arrhenius expressions, k2 = (1.8 +/- 1.0) X 10(-11) exp(-(1090 +/-299)/T) and k3 = (1.4 +/- 0.8) x 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.