Theoretical Study On Free Radicals Of DMPO Capture Dichlorobenzoquinone And Tert - Butyl Hydrogen Peroxide And Hydroxamic Acid System

As one of the important genotoxic and carcinogenic quinoid metabolites of pentachlorophenol(PCP), polyhalogenated quinones can create a variety of hazardous effects in vivo. Recently, more studies have shown that halogenated quinones can react with the hydrogen peroxide or organic hydroperoxides to produce the hydroxyl radical, organic alkoxyl radicals, and quinone ketoxy radicals experimentally, which can be used to partly explain the potential carcinogenicity of polyhalogenated aromatic environmental pollutants. In addition, the hydroxamic acids could markedly inhibit the formation of the hydroxyl or alkoxyl radicals in the reactions of halogenated quinones with hydrogen peroxide or organic hydroperoxides, exhibiting their potential applications in the detoxication of polyhalogenated quinones.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 used by the electron spin resonance(ESR) method experimentally. In this study, the spin trapping of 2-chloro-5-hydroxy-1,4-benzoquinone radical(CBQ) produced in the reaction of 2,5-dichloro-1,4-benzoquinone and t-butylhydroperoxide and N-methyl benzohydroxamic acid radical(?N-MeBHA) produced in the reaction of 2,5-dichloro-1,4-benzoquinone with N-methyl benzohydroxamic acid by 5,5-dimethyl-1-pyrroline N-oxide(DMPO) and its subsequent reaction processes have been systematically investigated at the B3LYP/6-311++G** level of theory in combination with the atoms in molecules(AIM) theory, natural bond orbital(NBO) theory, and ab initio molecular dynamics.This study is mainly divided into two parts to discuss:In the first part, the spin trapping of 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 subsequent reaction processes have been systematically discussed. It was shown 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 testified 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 CBQ fragment to the adjacent O atom of 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.In the second part, the spin trapping of N-methyl benzohydroxamic acid radical(?N-MeBHA) produced in the reaction of 2,5-dichloro-1,4-benzoquinone with N-methyl benzohydroxamic acid has been systematically investigated employing 5,5-dimethyl-1-pyrroline N-oxide(DMPO) as a spin trap. The tautomerization behavior of ?N-MeBHA radical has been firstly investigated and four tautomers including C- and N-centered forms have been located on its potential energy surface. After that, the nature of the formed spin adducts between DMPO and ?N-MeBHA radical 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(CBQ) deriving from DCBQ has been investigated as well as the keto-enol tautomerization behavior of the formed products. 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.