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.