Effects of substituent and external pressure on spin trapping rates of carbon dioxide anion, sulfur trioxide anion, hydroxyl, and ethyl radicals with various PBN- and DMPO-type spin traps

Using a competitive trapping method employing two different traps, the trapping rates of anionic radicals (CO₂^-· and SO₃^-·), hydroxyl, and ethyl were quantified for various PBN- and DMPO-type traps. We have examined the characteristic effects of substituent and external pressure on the spin trapping of anionic radicals and compared with those of neutral radicals such as ethyl and hydroxyl radicals. In DMPO-type traps, the effects of substituent on trapping rates of two anionic radicals were small compared with that of C₂H₅·. In PBN-type traps, the large effects of substituent on trapping rates of CO₂^-· and C₂H₅· were observed. Further, the activation volumes determined from the pressure dependence experiments for competitive reactions show a difference in pressure behavior between the two trapping reactions, and enable us to speculate on the reaction mechanism of substituted PBN and DMPO traps for anionic radicals.