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

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 K_2a=(2.7±0.4)×10^-19 cm³ was determined and a rate constant of k_2a=(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.