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.

Keywords:

• CIDEP
• EPR
• Hydroxyl radicals
• F-pairs
• Sulfate radical anion

Acknowledgements

We thank Professors P. J. Hore and R. Das for helpful discussions as this manuscript was being prepared for submission. This work was supported by the National Science Foundation through grant number CHE–0518300. P.C. thanks Fundacion Antorchas for a doctoral fellowship.