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

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 H 2 O 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 + CH 4 r̊ CH 3 + H 2 O and (2) OH + CF 3 H r̊ CF 3 + H 2 O. Rate constants were found to be k 1 (1300 K) = (2.5 ± 0.8) 10 12 and k 2 (1350 K) = (4.0 ± 1.0) 10 11 cm 3 /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 k 1 ( T ) = 10 6.19 ( T /K) 2.13 exp(‐1234 K/ T ) cm 3 /mol · s. The dependence of k 1 , on temperature is described in terms of transition state theory.