Electron Paramagnetic Resonance Tracks Condition-Sensitive Water Radical Cation

Oxidizing species or radicals generated in water are of vital importance in catalysis, the environment, and biology. In addition to several related reactive oxygen species, using electron paramagnetic resonance (EPR), we present a nontrapping chemical transformation pathway to track water radical cation (H₂O^+•) species, whose formation is very sensitive to the conditioning environments, such as light irradiation, mechanical action, and gas/chemical introduction. We reveal that H₂O^+• can oxidize the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to the crucial epoxy hydroxylamine (HDMP=O) intermediate, which further reacts with the hydroxyl radical (^•OH) for the formation of the EPR-active sextet radical (DMPO=O^•). Interestingly, we uncover that H₂O^+• can react with dimethyl methylphosphonate (DMMP), 2-methyl-2-nitrosopropane (MNP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), and α-phenyl-N-tert-butylnitrone (PBN) which contain a double-bond structure to produce corresponding derivatives as well. It is thus expected that both H₂O^+• and ^•OH are ubiquitous in nature and in various water-containing experimental systems. These findings provide a novel perspective on radicals for water redox chemistry.