Gas-phase reaction of the OH–benzene adduct with O2: reversibility and secondary formation of HO2

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Abstract
The reaction of OH radicals with benzene and consecutive reactions of benzene–OH adducts with O2 were studied in the gas phase in N2–O2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 nm photolysis of H2O2. Time-resolved detection of both OH and benzene–OH adducts was performed by continuous-wave (cw) UV-laser long-path absorption at around 308 nm. The reaction: OH+benzene→products [reaction (1)] was not affected by the presence of O2. Rate constants k1=(1.10±0.07)×10-12 cm3 s-1 and (1.06±0.07)×10-12 cm3 s-1 were obtained in N2 and O2, respectively. In N2 addition of NO2 did not change k1, from which an upper limit of 5% is derived for formation of H atoms in reaction (1). An absorption cross-section of σ(308 nm)=(5.8±1.5)×10-18 cm2 and a self-reaction rate constant of (3.4±1.7)×10-11 cm3 s-1 were determined for the benzene–OH adduct. Upper limits of 5×10-15 cm3 s-1, 1×10-14 cm3 s-1 and 5×10-14 cm3 s-1 were obtained for reactions of the adduct with benzene, H2O2 and NO, respectively. The adduct kinetics in the presence of O2 is consistent with the reversible formation of a peroxy radical: adduct+O2↔adduct–O2 [reaction (2a/-2a)]. An equilibrium constant of K2a=(2.7±0.4)×10-19 cm3 was determined and a rate constant of k2a=(2±1)×10-15 cm3 s-1 was roughly estimated. The effective adduct loss from the equilibrium can be explained by (i) an additional irreversible reaction of the adduct with O2 with a rate constant of (2.1±0.2)×10-16 cm3 s-1, or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (7.6±0.8)×102 s-1. For a reaction of the peroxy radical with O2 an upper limit of 1×10-17 cm3 s-1 is estimated. Addition of NO reveals formation of HO2 in the presence of O2 by recovering OH via HO2+NO. Applying numerical methods, reaction models were tested to describe the observed complex kinetics of OH. The data are consistent with rapid HO2 formation following a peroxy radical+NO reaction with a rate constant of (1.1±0.4)×10-11 cm3 s-1. Extrapolation of HO2 formation rates to [NO]=0 points at a second source of HO2 not preceded by any RO2+NO reaction.

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