A Combined Flash Photolysis/Shock‐Tube Study of the Absolute Rate Constants for Reactions of the Hydroxyl Radical with CH4 and CF3H around 1300 K

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Abstract
A combined flash photolysis/shock‐tube technique for the direct investigation of the rates of bimolecular reactions of the OH radical at temperatures between 1000‐1500 K is described. In this technique shock heated mixtures of approximately 0.5% of H2O in Ar and containing small amounts of a selected reactant, are subjected to flash photolysis as an instantaneous source of OH. The shock wave only serves to heat the gas mixture; thermal decomposition does not occur. Formation and subsequent decay of OH are monitored by time resolved UV resonance absorption. – First results with this technique have been obtained for the reactions (1) OH + CH4 r̊ CH3 + H2O and (2) OH + CF3H r̊ CF3 + H2O. Rate constants were found to be k1 (1300 K) = (2.5 ± 0.8) 1012 and k2 (1350 K) = (4.0 ± 1.0) 1011 cm3/mol · s. The result for reaction (1) is shown to confirm previously suggested non‐Arrhenius behaviour for this reaction, and in combination with low temperature data yields k1(T) = 106.19 (T/K)2.13 exp(‐1234 K/T) cm3/mol · s. The dependence of k1, on temperature is described in terms of transition state theory.