Density Functional Theory Study of Semiquinone Radical Anions of Polychlorinated Biphenyls in the Syn- and Anti-like Conformation

Polychlorinated biphenyls (PCBs) can be metabolized to reactive metabolites, such as PCB semiquinone radical anions (SQ^•–), whose structure and role in PCB-induced toxicity are difficult to investigate due to their relative instability. The unrestricted UB3LYP/6-311G** method was used to investigate several molecular descriptors of the syn- and anti-like conformation of SQs^•–. The bond lengths and angles of the quinone moiety of the SQs^•– were in between the values reported for PCB quinones and hydroquinones, which is consistent with the distribution of the α highest occupied molecular orbital (α-HOMO). The dihedral angles between the two ring systems increased in the presence of ortho chlorine substituents and were smaller compared to the corresponding PCB quinones. The ground-state energies indicate that the anti-like conformation of the SQs^•– is more favorable than the syn-like conformation. Molecular descriptor used for modeling of quantitative structure–activity relationships displayed some dependence on the conformation. These findings suggest that SQs^•– in both the syn- and antilike conformation may interact differently with target molecules, which may have implications for the toxicity of PCBs.