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Abstract

The reversible reaction of OH radicals with 1,2,4,5-tetramethylbenzene (1245-TeMB, durene) leads to adducts at the substituted (ipso) and unsubstituted (ortho) positions of the ring. By the use of flash photolysis for production and resonance fluorescence for detection of OH, the gas-phase reactions of O2 with these adducts were investigated over the temperature range of 300–340 K in He at 200 mbar. The decay of OH, generated by pulsed vacuum-UV photolysis of H2O, was monitored under slow-flow conditions in the presence of 1245-TeMB and O2 at concentrations of up to 19 × 1012 cm–3 and 2 × 1016 cm–3, respectively. Triexponential OH decays resulted from the unimolecular decomposition of the two adducts, representing OH reservoirs with different stabilities. In the presence of O2, additional adduct loss pathways exist, leading to faster OH consumption. Triexponential functions fitted to these decays were analyzed to obtain rate constants for the reactions of O2 with both adducts. Rate constants in the range of (4–13) × 10–15 and (0.3–3) × 10–15 cm3 s–1 were obtained for the ortho and the ipso adducts, respectively, depending on temperature and assumptions regarding details of the underlying mechanism of adduct isomer formation and isomerization. At O2 concentrations exceeding about 1 × 1016 cm–3, deviations from a linear dependence of the adduct loss rates on the O2 concentration indicate an even more complex mechanism. The validity of the rate constants is therefore confined to O2 concentrations below 1 × 1016 cm–3. The adduct + O2 rate constants for 1245-TeMB are greater than the corresponding previously obtained rate constants for benzene, toluene, and p- and m-xylene but smaller than those for hexamethylbenzene. The results are discussed in terms of the current knowledge about the mechanism of OH-induced degradation of aromatic compounds in the presence of O2.