Direct electron paramagnetic resonance evidence for ferryl ion (Fe^IVO²⁺) in Fenton chemistry

This study reports the first specific and direct identification of the ferryl ion (Fe^IVO²⁺) in Fenton reaction systems using electron paramagnetic resonance (EPR) spectroscopy. By integrating high-resolution mass spectrometry data and theoretical computational analyses, we demonstrate that the conventional spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) is selectively oxidized by Fe^IVO²⁺ via a multi-step reaction pathway, ultimately yielding 5,5-dimethyl-1-pyrrolidone-N-oxyl (DMPOX) as a characteristic product. The elucidated oxidation pathway involves key intermediates including DMPO-^•OH and 1-hydroxy-5,5-dimethyl-1-pyrrolid-2-one (HDMPN). The unique EPR spectral signature of DMPOX serves as a highly sensitive and unequivocal indicator of Fe^IVO²⁺, as verified through the application of the phenyl methyl sulfoxide (PMSO) probe technique, a widely employed method for the quantification of high-valent metal species. Further experimental validation confirms that this proposed approach effectively differentiates Fe^IVO²⁺ from hydroxyl radicals (^•OH) and enables quantitative monitoring of Fe^IVO²⁺ generation across ten iron-ligand complexes through modulation of the DMPO concentration. Therefore, this study provides a robust EPR-based diagnostic methodology to resolve the enduring controversy regarding the identification of the principal reactive species (^•OH and/or Fe^IVO²⁺) involved in the Fenton reaction, a subject that has been extensively examined for over a century.