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+center dot) species, whose formation is very sensitive to the conditioning environments, such as light irradiation, mechanical action, and gas/chemical introduction. We reveal that H2O+center dot 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)-O-center dot) for the formation of the EPR-active sextet radical (DMPO=O-center dot). Interestingly, we uncover that H2O+center dot can react with dimethyl methylphosphonate (DMMP), 2-methyl-2-nitrosopropane (MNP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), and alpha-phenyl-N-tert-butylnitrone (PBN) which contain a double-bond structure to produce corresponding derivatives as well. It is thus expected that both H2O+center dot and (OH)-O-center dot are ubiquitous in nature and in various water-containing experimental systems. These findings provide a novel perspective on radicals for water redox chemistry.

Automated summary

Oxidizing species or radicals generated in water play a vital role in catalysis, the environment, and biology. This study presents a nontrapping chemical transformation pathway to track the formation of water radical cation (H2O+center dot) species, which is sensitive to environmental factors. The researchers discovered that H2O+center dot can react with various compounds, leading to the formation of important intermediates and derivatives. These findings provide a new perspective on radicals in water redox chemistry.