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 (a_N = 12.65 G; a^β_β = 10.4G; a_γ = 1.3G); heptane (a_N = 12.49G; a_β^β = 10.29G; a^γ_H = 1.2g); and acetone (a_N = 12.6G; a_β^β = 10.17 G; a_γ = 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 a_N and a_β^β both decrease by about 1 G on going from water to ethanol, further decreases in polarity do not greatly affect these EPR parameters.