High-temperature reactions of OH radicals with benzene and toluene.

The rate constants for the reactions of OH radicals with benzene and toluene have been measured directly by a shock tube/pulsed laser-induced fluorescence imaging method at high temperatures. The OH radicals were generated by the thermal decomposition of nitric acid or tert-butyl hydroperoxide. The derived Arrhenius expressions for the rate constants were k(OH + benzene) = 8.0 x 10(-11) exp(-26.6 kJ mol(-1)/RT) [908-1736 K] and k(OH + toluene) = 8.9 x 10(-11) exp(-19.7 kJ mol(-1)/RT) [919-1481 K] in the units of cubic centimeters per molecule per second. Transition-state theory (TST) calculations based on quantum chemically predicted energetics confirmed the dominance of the H-atom abstraction channel for OH + benzene and the methyl-H abstraction channel for OH + toluene in the experimental temperature range. The TST calculation indicated that the anharmonicity of the C-H-O bending vibrations of the transition states is essential to reproduce the observed rate constants. Possible implications to the other analogous H-transfer reactions were discussed.