Gas-Phase Reaction Kinetics of the Ortho and Ipso Adducts 1,2,4,5-Tetramethylbenzene–OH with O₂

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.