Kinetics and Thermochemistry of the Hydroxycyclohexadienyl Radical Reaction with O 2 : C 6 H 6 OH + O 2 ⇌ C 6 H 6 (OH)OO

The UV absorption spectrum along with the self-reaction and oxidation reaction kinetics of the hydroxycyclohexadienyl radical, C 6 H 6 OH (which results from OH addition to benzene), were studied using excimer laser photolysis coupled to transient UV absorption. The radicals were generated by photolysis of N 2 O/H 2 O/C 6 H 6 /He mixtures at 193 nm in a series of chemical reactions initiated by O( 1 D). The radical has continuous absorption in the range 260−340 nm with a maximum absorption cross-section of (8.1 ± 1.4) × 10 -18 cm 2 molecule -1 at 280 nm. Reaction of the radical with molecular oxygen, C 6 H 6 OH + O 2 ⇌ C 6 H 6 (OH)OO (1), and self-reaction C 6 H 6 OH + C 6 H 6 OH → products (2), were studied over the 252−285 K temperature range at 1.01 ± 0.02 bar (He). The radical temporal profiles were recorded via transient absorption at 315 nm. In reaction 1, two-time-domain “equilibration” kinetics were recorded in the temperature range 252−273 K. The rate constant of the addition reaction is k 1 = (1.4 ± 0.8) × 10 -12 exp(−18.6 ± 1.7 kJ mol -1 / RT ) cm 3 molecule -1 s -1 . The standard enthalpy of reaction 1 was determined from the measured equilibrium constants using the third law method: = −43.6 ± 2.0 kJ mol -1 . The measured rate constant of self-reaction 2 is k 2 = (6 ± 3) × 10 -11 exp(−2.00 ± 1.6 kJ mol -1 / RT ) cm 3 molecule -1 s -1 .