The reaction of the hydroxycyclohexadienyl radical (HO-C 6 H 6 ) (the adduct from the benzene + OH reaction) with O 2 has been investigated using laser flash photolysis with UV-absorption spectroscopic detection, and DFT and ab initio quantum mechanical calculations. An absolute absorption spectrum was measured for the benzene-OH adduct, and its reaction with O 2 , giving a peroxy radical species, was seen to be equilibrated around room-temperature. An equilibrium constant of 1.15 AE 0.6 Â 10 À19 cm 3 molecule À1 was determined at 295 K from an analysis of transient absorption signals using a detailed reaction mechanism. Equilibrium constants were obtained in this way at six different temperatures between 265 and 345 K. The temperaturedependence of these data indicates that the DH 0 298 and DS 0 298 for the title reaction are À10.5 AE 1.3 kcal mol À1 and À33.9 AE 1.4 cal K À1 mol À1 respectively (second-law analysis of the data, 2s errors). A third-law analysis of the data (using a value for DS 0 298 of À38.3 cal K À1 mol À1 , derived from DFT and ab initio calculations) yields a value for DH 0 298 of À11.7 AE 0.2 kcal mol À1 , which compares with an ab initio calculated value of À12.2 kcal mol À1 . Absorption signals at 260-275 nm, in the presence of high concentrations of O 2 , were observed that are consistent with the presence of the benzene-OH peroxy radical, and with stable products of its chemistry. Equilibrium constants obtained from these data agree well with our other determinations. The effective lifetime of the equilibrium system-adduct + O 2 Ð adduct À O 2 -is dictated either by an additional, irreversible reaction of the benzene-OH adduct with O 2 or by a unimolecular transformation of the peroxy species. Assuming the former case, a bimolecular rate constant of around 5.5 AE 3.0 Â 10 À16 cm 3 molecule À1 s À1 was estimated from a kinetic simulation of our decay signals. This rate constant does not appear to vary significantly between 265 and 320 K, but it must be emphasised that it was estimated with a fairly high uncertainty.
The kinetics and mechanism of the gas-phase reaction of the cyclohexadienyl radical c-C 6 H 7 with O 2 have been investigated using both experimental and theoretical approaches. The rate constant has been measured using conventional flash photolysis in the temperature range 302-456 K, 1 atm pressure. c-C 6 H 7 radicals were produced by reacting Cl atoms with 1,4-cyclohexadiene. The rate expression is k 1 ¼ (1.4 AE 0.26) Â 10 À13 exp[À(300 AE 74) K/T] cm 3 molecule À1 s À1 (2s error bars). The reaction can proceed either by association, yielding a peroxy radical RO 2 or by H-abstraction, yielding benzene + HO 2 , the two reaction channels involving two distinct transition states. In contrast to what is observed for the c-C 6 H 6 OH radical, no equilibrium with the peroxy radical could be characterised. The theoretical approach, involving both DFT and ab initio methods, was used to determine if the measured rate constant should be assigned to the association or to the H-abstraction channel. Comparison of experimental and theoretical results shows that H-abstraction must be the only significant reaction channel.
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