A SPIN-TRAPPING STUDY OF THE PHOTOCHEMISTRY OF 5,5-DIMETHYL-1-PYRROLINE N-OXIDE (DMPO)

The photochemistry of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been studied in benzene, cyclohexane and aqueous buffer solutions (pH 7.4) by means of electron paramagnetic resonance (EPR) and the spin trappi ng technique. Ultraviolet irradiation of DMPO in aqueous buffer with u nfiltered UV radiation from a Xe are lamp results in photoionization o f the spin trap and the generation of the DMPO cation radical, DMPO(+) . The aqueous electron, e(aq)(-), was trapped by DMPO and detected as the DMPO/H. adduct. The DMPO(+). reacted with the water to yield the D MPO/.OH adduct. Ultraviolet irradiation of DMPO in nitrogen-saturated benzene gave an unidentified carbon-centered DMPO adduct that was repl aced by hydroperoxyl and alkoxyl adducts of DMPO when oxygen was prese nt. Experiments employing O-17(2) gas indicated that the oxygen in the DMPO alkoxyl adduct was derived from molecular oxygen. However, UV ir radiation of DMPO in cyclohexane yielded the cyclohexyl and cyclohexyl oxyl adducts of DMPO in nitrogen-saturated and air-saturated solutions , respectively. These observations suggest that in aprotic solvents UV irradiation of DMPO generates a carbon-centered radical (R.), derived from the trap itself, which in benzene reacts with oxygen to yield an alkoxyl radical(RO.), possibly via a peroxyl radical (ROO.) intermedi ate. In cyclohexane R. abstracts a hydrogen atom from the solvent to y ield the cyclohexyl radical in the absence of oxygen and the cyclohexy loxyl radical in the presence of oxygen. These findings indicate that when DMPO is used as a spin trap in studies employing short-wavelength UV radiation (lambda < 300 nm) the photochemistry of DMPO cannot be i gnored.