Rearrangement pathways of arylperoxy radicals. 2. Five-membered heterocycles

The potential energy surfaces for the reaction of furanyl and oxazolyl radi cals with O-2 have been examined using the B3LYP method. The initial produc tion of the arylperoxy radical followed by either simple decomposition or r earrangement to yield several intermediates (aryloxy, dioxiranylaryl, or di oxetanylaryl radicals) has been explored. Transition state structures for m ost of the steps are presented as well as relative free energies over a ran ge of temperatures from 298 to 2000 K. The energetics of the analogous inte rmediates for the reaction of O-2 and other five-membered heterocyclic radi cals 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 O- 2 loss at temperatures less than or equal to 500 K and favored in the same temperature range over O atom loss in several cases. Dioxetanyl formation i ncurs the greatest barrier to formation, and direct routes are not availabl e in every molecule surveyed. However, in some cases the dioxetane radicals transform rapidly into very stable species.