Rearrangement pathways of arylperoxy radicals. I. The azabenzenes

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 rearrang ement to yield the intermediates (pyridinyloxy, dioxiranylpyridinyl, or dio xetanylpyridinyl radicals) has been explored. Transition-state structures f or 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 dissociat ion from the arylperoxy radical is preferred rather than the loss of O atom to generate the corresponding aryloxy radical, and this preference is cont rary to phenylperoxy radical decomposition. However, the formation of a dio xiranyl radical intermediate is the most accessible intermediate from the p eroxy precursor at temperatures less than or equal to 500 K. Dioxetanyl int ermediates are less favored but may provide a route to NOx generation from nitrogen substitution in aromatic fuels.