Substituent effects in the kinetic analysis of free radical reactions with nitrone spin traps

The kinetics of reactions of the water radiolysis species with substituted phenyl-N-tert-butyl nitrones (PBN), have been studied directly, in situ, using a pulsed Van de Graaff accelerator and kinetic spectrophotometry. The spin trapping of other secondary radiation-induced electrophilic and nucleophilic free radical species was also studied. The mechanistic aspects of these reactions are discussed in terms of the kinetic data. The effect of structure and substituents on the rate of spin trapping was tested for five para-substituted PBN's. It was found that electron-withdrawing groups increase the rate of nucleophilic reactions involving hydrated electrons, formyl anions, and α-hydroxyalkyl radicals with substitued PBN. Electron-donating substituents produce the opposite effect. These studies confirm that nitrones are effective spin traps because of their high reactivity towards radiation-induced free radicals. Caution must be observed, however, in view of their lack of specificity, in interpreting spin-trapping data in complex steady-state systems involving more than one primary or secondary radical species, or where unstable spin-adducts are formed.