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Benzoxyl Radical Decomposition Kinetics: Formation of Benzaldehyde + H, Phenyl + CH2O, and Benzene + HCO

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journal contribution
posted on 25.06.2009, 00:00 by Gabriel da Silva, Joseph W. Bozzelli
The kinetics of benzoxyl radical decomposition was studied using ab initio computational chemistry and RRKM rate theory. The benzoxyl radical is an important but short-lived intermediate in the combustion of toluene and other alkylated aromatic hydrocarbons. A theoretical study of the thermochemistry and kinetics to products over a range of temperatures and pressures for benzoxyl decomposition is reported. Ab initio calculations with the G3X theoretical method reveal low-energy pathways from the benzoxyl radical to benzaldehyde + H and the phenyl radical + formaldehyde (CH2O), as well as a novel mechanism to benzene + the formyl radical (HCO). RRKM simulations were performed for benzoxyl decomposition as a function of temperature and pressure. Benzaldehyde formation constitutes more than 80% of the total reaction products at temperatures below 1000 K, decreasing to around 50% at 2000 K. Formation of benzene + HCO and phenyl + CH2O is of similar importance, each accounting for 5−10% of the decomposition products at around 1000 K, increasing to 20−30% at 2000 K. The results presented here should lead to improved kinetic models for the oxidation of alkylated aromatic hydrocarbons, particularly for the formation of benzene as a direct oxidation product of toluene. Re-evaluation of the phenyl radical heat of formation leads us to suggest a benzene C−H bond dissociation energy in the range of 113.5−114.5 kcal mol−1.