An experimental and theoretical investigation of the gas-phase benzene–OH radical adduct + O2 reaction

Abstract

The reaction of the hydroxycyclohexadienyl radical (HO–C₆H₆) (the adduct from the benzene + OH reaction) with O₂ 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 O₂, giving a peroxy radical species, was seen to be equilibrated around room-temperature. An equilibrium constant of 1.15 ± 0.6 × 10⁻¹⁹ cm³ molecule⁻¹ 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 temperature-dependence of these data indicates that the ΔH⁰₂₉₈ and ΔS⁰₂₉₈ for the title reaction are −10.5 ± 1.3 kcal mol⁻¹ and −33.9 ± 1.4 cal K⁻¹ mol⁻¹ respectively (second-law analysis of the data, 2σ errors). A third-law analysis of the data (using a value for ΔS⁰₂₉₈ of −38.3 cal K⁻¹ mol⁻¹, derived from DFT and ab initio calculations) yields a value for ΔH⁰₂₉₈ of −11.7 ± 0.2 kcal mol⁻¹, which compares with an ab initio calculated value of −12.2 kcal mol⁻¹. Absorption signals at 260–275 nm, in the presence of high concentrations of O₂, 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 + O₂ ⇌ adduct − O₂—is dictated either by an additional, irreversible reaction of the benzene–OH adduct with O₂ or by a unimolecular transformation of the peroxy species. Assuming the former case, a bimolecular rate constant of around 5.5 ± 3.0 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ 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.