Spin-Trapping of the Superoxide Radical in Aprotic Solvents

The superoxide adduct of 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) has been detected by EPR spectroscopy in aprotic solvents using KO2 solubilized in 18-crown-6-ether as a source of superoxide. The EPR hyperfine splitting constants of the DMPO-superoxide adduct were as follows: benzene/toluene (aN = 12.65 G; aH beta = 10.4 G; aH gamma = 1.3 G); heptane (aN = 12.49 G; aH beta = 10.29 G; aH gamma = 1.2 g); and acetone (aN = 12.6 G; aH beta = 10.17 G; aH gamma = 1.3 G). The EPR parameters for benzene, toluene and heptane differ significantly from previously reported values. A plot of the hyperfine splitting constants for the DMPO superoxide adduct as a function of solvent polarity (Kosower Z value) indicates that while aN and aH beta both decrease by about 1G on going from water to ethanol, further decreases in polarity do not greatly affect these EPR parameters.