On the electron spin polarization observed in TREPR experiments involving hydroxyl and sulfate radicals

Abstract

Hydrogen peroxide and the persulfate dianion both undergo photochemical cleavage reactions in aqueous solution, creating a pair of hydroxyl (HO•) or sulfate anion () radicals, respectively. These radicals can abstract H-atoms or undergo electron transfer reactions in the presence of organic substrates, leading to carbon-centred free radicals. The chemically induced electron spin polarization (CIDEP) pattern created during time-resolved electron paramagnetic resonance (TREPR) experiments involving these species is always observed to be low field emissive/high field absorptive (E/A), coupled with net absorption. There are numerous suggestions in the literature regarding the origin of the E/A* pattern: Contributions from thermally equilibrated radicals, concentration-dependent spin relaxation, or the result of a spin–selective electron transfer reaction to create closed shell ionic structures. Here, it is proposed that E/A and E/A* contributions arise from two different F-pairs that must be present in these solutions. Examples are presented and the origin of the effect is explained in terms of well-established CIDEP theories for freely diffusing radical pairs. The importance of the A* contribution is discussed with regard to the recently reported observation of peroxyl radicals from addition of oxygen to amino acid radicals at room temperature.