Non-Arrhenius behavior in bimolecular reactions of the hydroxyl radical.
Rate measurements over a very wide temp. range (300-2000 K) for some reactions involving the OH radical show distinct non-Arrhenius behavior. An attempt to fit the available data empirically leads to k1 = 106.19T2.13 exp(-1233 K/T) cm3/mol-s and k2 = 108.0T1.6 exp(-1660K/T) cm3/mol-s for the reactions with a finite activation barrier around room temp. ((1) OH + CH4 -> CH3 + H2O and (2) OH + H2 -> H2O + H) and k3 = exp(24.98 + 9.2 * 10-4T) cm3/mol-s and k4 = exp(27.1 + 1.5 * 10-3T) cm3/mol-s for reactions without substantial temp. dependence around 300 K ((3) OH + CO -> CO2 + H and (4) OH + OH -> H2O + O). The extent of non-Arrhenius behavior may plausibly be explained by means of the total reactive cross section, for which various forms are derived. A more direct explanation, however, is provided from detns. of the vibrational-rate enhancement. For OH + H2, vibrational excitation of H produces sufficient enhancement of the rate const. kOH+H2(v = 1)/kOH+H2(v = 0) = 1.5(-3.5+1.3)102 at 298 K to account for the Arrhenius graph curvature of the overall thermal rate const.
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