Ultraviolet photolysis of monochloro-p-benzoquinone (MCBQ) in aqueous solution: Theoretical investigation into the dechlorination

In this work, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution has been systematically investigated through quantum chemical calculations. During the UV irradiation at 253.7 nm, the first triplet state of MCBQ ((3)MCBQ*) was from the intersystem crossing of its first excited singlet state ((1)MCBQ*). In aqueous solution, the nucleophilic attack of OH- on carbon atoms in (3)MCBQ* was the central reaction. The addition of OH- to olefinic carbon atoms was much more kinetically feasible than that to carbonyl carbon atoms, even though the carbonyl carbon atoms were more positively charged. Moreover, OH- preferred to add to the ortho-position of C-Cl bond, where the unchlorinated atom was more negatively charged than the chlorinated one. The UV photolysis of the primary intermediate (HO-CBQ) was not the same as that of MCBQ. The attack of OH- on the para-position of C-Cl bond was the most efficient pathway. The addition of OH- to the chlorinated atom of (HO)-H-3-CBQ* was much more efficient than that in the case of (3)MCBQ*, which reveals that more UV irradiation may promote the dechlorination. The findings in the present study may be helpful to enrich the understanding of the halobenzoquinones transformation in aqueous solution.

Automated summary

In this study, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution was systematically investigated through quantum chemical calculations. It was found that the nucleophilic attack of OH- on carbon atoms was the central reaction, and the addition to olefinic carbon atoms was more feasible. Moreover, OH- preferred to add to the ortho-position of C-Cl bond.