An identification of the C–C bonding spin adduct in the spin trapping of N-methyl benzohydroxamic acid radical by 5,5-dimethyl-1-pyrroline N-oxide

A detailed knowledge of the spin trapping of the radicals by spin traps is crucial for the elucidation of the reaction mechanisms involving radicals and the rational design of the novel efficient spin traps experimentally. In this study, the spin trapping of N-methyl benzohydroxamic acid radical (·N-MeBHA) produced in the reaction of 2,5-dichloro-1,4-benzoquinone (DCBQ) with N-methyl benzohydroxamic acid has been systematically investigated employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap theoretically. The tautomerization behavior of ·N-MeBHA radical has been firstly investigated, and four tautomers including C- and N-centered forms have been located. After that, the nature of the formed spin adducts has been explored as well as the thermodynamic and kinetic parameters associated with the spin-trapping process. Moreover, the reaction of the ·N-MeBHA radical with the C-centered quinone ketoxy radical has been studied. Besides the available C–N bonding product identified experimentally, more stable C–C bonding products have also been observed. Additionally, significant catalytic role of explicit water molecules should be highlighted in the tautomerization reaction of the ·N-MeBHA radical and the keto–enol tautomerization reaction of the final products. This study demonstrates for the first time the possibility of the existences of the C-centered ·N-MeBHA radical and the C–C bonding product in the reaction of DCBQ and N-MeBHA, providing new insights into the reaction mechanisms between polyhalogenated quinones and hydroxamic acids.