On the Reaction Mechanism of Nitrate Radical and DMPO in Non-aqueous Photocatalytic Media

DMPO (5,5-dimethyl-1-pyrroline N-oxide), a widely employed spin-trapping agent, is selectively oxidized by nitrate radicals (NO₃ ^•) generated in irradiated, nonaqueous TiO₂ suspensions, yielding DMPOX, a relatively stable species with a distinctive EPR spectrum. DMPOX exhibits highly characteristic spectral features, enabling its use as a semi-quantitative fingerprint for the in situ detection of NO₃ ^•. This reaction occurs in various aprotic solvents, with acetonitrile demonstrating suitable stability under irradiation and selected for further investigation. Mechanistic analysis based on DFT calculations at the M06-2X/def2-TZVPD level reveals that the most favorable pathway involves the electrophilic addition of NO₃ ^• to the unsaturated carbon of the C═N⁺-O^- fragment, followed by hydrogen atom abstraction (HAT) by a second NO₃ ^• and subsequent NO₂ ^• elimination to form 2-oxo-5,5-dimethyl-1-pyrroline 1-oxyl (DMPOX). These results highlight the utility of this system for probing nitrate radical formation as a function of the intrinsic photocatalyst characteristics, providing a robust diagnostic platform to support the deployment of nitrate radicals in heterogeneous photocatalytic applications.