The potential energy surface for the unimolecular decomposition of phenylperoxy radical has been explored using the B3LYP method. Several pathways were considered including the initial formation of the phenoxy, dioxiranyl, 1,2-dioxetanyl, 1,3-peroxy, and p-phenylquinone radicals. Transition states for all pathways on the potential energy surface are presented. At all temperatures studied (T e 1250 K), the energetically most favored pathway is the dioxiranyl pathway which leads to the formation of cyclopentadienyl radical and CO 2 , pyranyl radical and CO, or an acyclic C 6 H 5 O 2 radical structure as products. The ring-opening reactions are very competitive with formation of CO and CO 2 as products.

The potential energy surface (PES) of the C6H5 • + O2(3Σg) reaction has been studied using the B3LYP method. Several pathways were considered following the formation of the phenylperoxy (C6H5OO•) radical. At low temperatures (T < 432 K), the lowest energy pathway was found to go through a dioxiranyl radical. Scission of the O−O bond to form the phenoxy (C6H5O•) radical and O(3P) atom is more favorable at higher temperatures. Transition state structures for several steps in the decomposition of the phenylperoxy radical are presented to augment the C6H5 • + O2 PES. For the heteroatomic aromatic hydrocarbon radicals, such as pyridine, furan, and thiophene, only minima on the PES are calculated in analogy with the intermediates obtained for the reaction of phenyl radical with O2. One important result of the proposed decomposition mechanism is that subsequent rearrangements of the heteroatomic aromatic hydrocarbon peroxy radicals (Ar−OO•) are likely to yield intermediates that are of atmospheric interest.

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 C4, C3, and C2 compounds. The decomposition of 2-oxepinoxy radical for these pathways provides a mechanism that proceeds from C6 → C5 → C4 → C2 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-cyclopenten-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.

The potential energy surfaces for the reaction of pyridinyl radicals with O 2 have been studied using the B3LYP method. The initial production of the pyridinylperoxy radical followed by either simple decomposition or rearrangement to yield the intermediates (pyridinyloxy, dioxiranylpyridinyl, or dioxetanylpyridinyl radicals) has been explored. Transition-state structures for most of the steps are presented as well as relative free energies over a range of temperatures from 298 to 2000 K. The energetics of the analogous intermediates for the reaction of O 2 and other azabenzene radicals derived from pyridazine, pyrimidine, and pyrazine are also provided. O 2 dissociation from the arylperoxy radical is preferred rather than the loss of O atom to generate the corresponding aryloxy radical, and this preference is contrary to phenylperoxy radical decomposition. However, the formation of a dioxiranyl radical intermediate is the most accessible intermediate from the peroxy precursor at temperatures e500 K. Dioxetanyl intermediates are less favored but may provide a route to NO x generation from nitrogen substitution in aromatic fuels.

The potential energy surfaces for the reaction of furanyl and oxazolyl radicals with O2 have been examined using the B3LYP method. The initial production of the arylperoxy radical followed by either simple decomposition or rearrangement to yield several intermediates (aryloxy, dioxiranylaryl, or dioxetanylaryl radicals) has been explored. Transition state structures for most of the steps are presented as well as relative free energies over a range of temperatures from 298 to 2000 K. The energetics of the analogous intermediates for the reaction of O2 and other five-membered heterocyclic radicals derived from pyrrole and thiophene are also provided. The loss of an O atom is generally the most accessible and energetically favored pathway of decomposition at all temperatures. Dioxiranyl formation is favored over O2 loss at temperatures ≤500 K and favored in the same temperature range over O atom loss in several cases. Dioxetanyl formation incurs the greatest barrier to formation, and direct routes are not available in every molecule surveyed. However, in some cases the dioxetane radicals transform rapidly into very stable species.