The reaction of phenyl radical with molecular oxygen: A G2M study of the potential energy surface

Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C₆H₅ with O ₂. The reaction is shown to start with an exothermic barrierless addition of O₂ to the radical site of C₆H₅ to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[2.5]octadienyl (dioxiranyl, 8) radicals. Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8. The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C₅H₅ + CO₂, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C ₂H₂. Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8. Thus, the branching ratio of C ₆H₅O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10. According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C₅H₅ + CO₂, followed by pyranyl + CO and o-benzoquinone + H. Since C₆H₅O + O and C₅H₅ + CO₂ are expected to be the major primary products of the C₆H₅ + O₂ reaction and thermal decomposition of C₆H₅O leads to C₅H ₅ + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring. Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions.