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.

Ab initio calculations at different levels of theory have been performed for the title H-abstraction reactions. Total energies at stationary points of the potential energy surfaces for the reaction systems were obtained at MP2 and MP4 levels and improved by using Gaussian-2 (G2) methodology. The calculated G2 heats of reaction agree well with the experimental ones for both methoxy (product resulting from hydroxyl-side attack) and hydroxymethyl (product resulting from methyl-side attack) reaction channels. Calculations of the potential energy surfaces for the reaction systems show that H-abstraction from methanol by H, CH 3 , and OH (for methoxy reaction channel) proceeds by simple metathesis. The mechanism of the hydroxymethyl channel of reaction CH 3 OH + OH appears to be more complex, and it may consist of two consecutive processes. The reaction rate is determined by the energy barrier of the first process. Differences in the heights of the calculated energy barriers explain the differences in the reactivity of H, CH 3 , and OH toward methanol. The calculated barriers indicate a significant dominance of the hydroxymethyl formation channel for the CH 3 OH + H and CH 3 OH + OH reaction systems. Rationalization of the derived energy barriers has been made in terms of the polar effect. The calculated rate constants are in very good agreement with experiment and allow a description of the kinetics of the reactions under investigation in a wide temperature range with the precision that is required by practical applications such as modeling of the chemistry of methanol combustion.

We experimentally determined complete fallo † curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two di †erent techniques, laser Ñash photolysis and fast Ñow reactor were used both coupled to a detection of radicals by laser induced Ñuorescence. Experiments were performed at total C 2 H 5 Op ressures between 0.001 and 60 bar of helium and in the temperature range of 391È471 K. Under these conditions the b-CÈC scission (1a)is the dominating decompositionFrom a complete analysis of the experimental fallo † curves the low and the high pressure limiting rate constants of 3.3 ] 10~8 exp([58.5 kJ mol~1/RT ) cm3 s~1 and exp([70.3 kJ k 1a,0 \ [He] k 1a,= \ 1.1 ] 1013 mol~1/RT ) s~1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ^30%. Preexponential factor and activation energy are signiÐcantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the b-CÈH scission (1b) of exp([84 kJ CH 3 CH 2 O~] M ] CH 3 CHO ] H~] M k 1b,= \ 1.3 ] 1013 mol~1/RT ) s~1. Atmospheric implications are discussed.

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