Rate measurements of reactions of OH by resonance absorption. Part 2.—Reactions of OH with CO, C 2 H 4 and C 2 H 2

The rates of the reactions of OH with CO, C2H4 and C2H2 have been determined between 210 K and 460 K using time-resolved resonance absorption to monitor the removal of OH radicals following their creation by flash photolysis of mixtures containing H2O or N2O + H2. At 300 K, the rate constant, k6, for OH + CO → CO2+ H (6) is 8.7 × 1010 cm3 mol–1 s–1; k6 shows a slight positive temperature dependence, but the Arrhenius plot appears to be slightly curved. The nature of the path of this reaction is discussed and the results of transition state calculations are shown to agree well with the experimental data and to predict marked curvature in the Arrhenius plot above 500 K. The rate constants, k9 and k10, for the primary reactions of OH with C2H4 and C2H2 also increase only very slowly with temperature but, in these cases, the experimental results for 210 K ⩽T⩽ 460 K do fit Arrhenius expressions (energies of activation in kJ mol–1): k9= 4.5 × 1012 exp– 0.9/RT cm3 mol–1 s–1. k10= 1.2 × 1012 exp– 2.1/RT cm3 mol–1 s–1.