Theoretical studies on the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical by 5,5-dimethyl-1-pyrroline N-oxide (DMPO): the identification of the C–O bonding spin adduct

Abstract

The detection and identification of related radicals is crucial for the elucidation of the reaction mechanisms for metal-independent decomposition of hydroperoxides by halogenated quinones. In this study, the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical (CBQ) produced in the reaction of 2,5-dichloro-1,4-benzoquinone and t-butylhydroperoxide by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and its subsequent reaction processes have been systematically investigated at the B3LYP/6-311++G(d,p) level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) theory, and ab initio molecular dynamics. It was found that DMPO and CBQ can not only form the C–C bonding spin adduct observed experimentally, but also can form the C–O bonding spin adduct. This point has been further tested by the spin trapping of the other halogenated CBQ radicals. After that, the keto–enol tautomerization occurs for the formed C–C bonding spin adduct, where the explicit water molecule plays an important catalytic role in assisting the proton transfer process. Subsequently, spontaneous proton transfer has been observed from the hydroxyl group of the CBQ fragment to the adjacent O atom of the DMPO fragment in the formation process of the oxidation state of the spin adduct. These results not only help deepen our understanding of the spin trapping mechanism of CBQ-type radicals by DMPO, but also can provide important clues to the clarification of the reaction mechanism between halogenated quinone and organic hydroperoxides.