Computational study of the oxygen initiated decomposition of 2-oxepinoxy radical: A key intermediate in the oxidation of benzene

Density functional theory was utilized to determine whether the addition of O-2((3)Sigma(g)) to 2-oxepinoxy radical, a proposed intermediate in the unimolecular decomposition of phenylperoxy radical, followed by unimolecular rearrangement and decomposition results in the formation of experimentally detected C-1-C-5 products via oxidative combustion of benzene. B3LYP/6-31G* geometries for possible pathways resulting from the initial formation of 1,2-dioxetanyl, 1,3-peroxy, 1,4-peroxy, hydroperoxy, and peroxy moiety scission intermediates were calculated. Energies were determined by B3LYP/6-311+G** single-point energy calculations on the B3LYP/6-31G* geometries. For the O-2 addition steps and most favored pathway, the B3LYP/6-31G* geometries were reoptimized and energies obtained via the CBS-QB3 method. The B3LYP/6-31G* geometries were also used to obtain the energetic parameters to generate the free energy profiles for all pathways at 298, 500, 750, 1000, and 1250 K. For temperatures between 298 and 750 K, the formation of peroxyoxepinone radicals and their decomposition pathways and products are competitive with those proposed by Fadden for the unimolecular decomposition of 2-oxepinoxy radical. However, a large entropic penalty, associated with the step for 02 addition to 2-oxepinoxy radical, is incurred at higher temperatures, thereby making these pathways less competitive as the temperature rises. At temperatures < 1250 K, the same pathway maintains the lowest overall free energy profile and corresponds to rearrangement of 6-peroxyoxepinone (1c) to form a 1,4-peroxy intermediate between the ring carbons adjacent to the ester moiety (15a), followed by release of CO2 to form 5-oxapentenal radical (21c), which then cyclizes (22c) and releases formyl radical, thereby generating furan, CO2, and formyl radical as final products (10a). At 1250 K, all pathways proceeding from 2-peroxyoxepinone (1a) and through the peroxy bond scission intermediate (16a) have the lowest free energy profile.