Unimolecular decomposition of the 2-oxepinoxy radical: A key seven-membered ring intermediate in the thermal oxidation of benzene

The Gibbs free energy profiles for the unimolecular decomposition of the 2-oxepinoxy radical at temperatures ranging from 298 to 1250 K have been obtained using the B3LYP method. Intermediates and transition states have been obtained that link the 2-oxepinoxy radical to various products that have been experimentally observed during the thermal oxidation of benzene. The pathways explored include the rearrangement of 2-oxepinoxy radical to cyclopentadienyl radical, pyranyl radical, and various C-4, C-3, and C-2 compounds. The decomposition of 2-oxepinoxy radical for these pathways provides a mechanism that proceeds from C-6 --> C-5 --> C-4 --> C-2 intermediates but does not include the cyclopentadienyl radical as a required intermediate. The most viable pathway at temperatures between 1000 and 1250 K yields either the 5-oxo-2-cyclopenten-1-yl radical and CO or the vinyl radical, acetylene, and two molecules of CO as products. A mechanism to form 5-oxo-2-cycloponten-1-yl, a possible precursor to cyclopentadienone, is compared to the pathway for cyclopentadienone formation from the decomposition of phenoxy radical to cyclopentadienyl radical and then subsequent oxidation and rearrangement.