Exploratory ab initio MO calculations on the structures of polychlorinated biphenyls(PCBs): a possible way to make a coplanar PCB stable at coplanar conformation

Ab initio MO calculations for all of 20 coplanar polychlorinated biphenyls (PCBs) and several selected non-coplanar PCBs are performed. From the results of geometry optimization, the characteristic torsional angles of stable conformation for both coplanar and non-coplanar PCBs are clarified. The shape of typical potential energy curve of the ground state of coplanar PCB is very different from those of non-coplanar PCBs. Non-trivial barrier height at coplanar conformation indicates that coplanar PCB of the ground state is not stable at coplanar geometry without external perturbation. We performed MO calculations using cluster model of coplanar PCB with hydration of water molecules. From our rigid-rotation analysis, it was found that hydration to coplanar PCB by only one or two water molecules can drastically decrease rotational barrier height at the coplanar conformation. Our result may suggest a possible way to make a coplanar PCB stable at coplanar geometry in biological systems. (C) 1999 Elsevier Science B.V. All rights reserved.