Quantum Chemical and RRKM Investigation of the Elementary Channels of the Reaction C6H6 + O (3P)

We performed a computational study of an important reaction in the combustion of hydrocarbons, C₆H₆, + O (³P), using ab initio and RRKM methods. Density functional theory (B3LYP) was used to optimize geometries and obtain molecular vibrational frequencies, and complete basis set extrapolation (CBS-QB3) was used to obtain the energies for the reactants several transition states and products. The initial formation of a stabilized adduct is characterized by a barrier of 4.9 kcal mol^-1, in good agreement with the measured activation energy for this reaction. All product channels originate from rearrangement or decomposition of this adduct, which our calculations suggest is a triplet ground state. All of our ab initio calculations are thus conducted on the triplet surface. There are several products that are energetically accessible at combustion temperatures, but the formation of phenoxy radical, C₆H₅O, and H atom dominate the product slate at low temperatures. Rearrangement to form formylcyclopentadiene, C₅H₅CHO, is also important at low temperatures, and the decomposition of this species to form cyclopentadienyl radical, C₅H₅, and HCO may be significant at higher temperatures. Rearrangement to form phenol is unimportant at all temperatures.