Theoretical considerations about chiral PCBs and their methylthio and methylsulfonyl metabolites being possibly present as stable enantiomers

Ortho-substituted polychlorinated biphenyls (PCBs) as well as derivatives which contain one methylthio or methylsulfonyl group (in ortho position as well) were studied by quantum chemical methods. The calculations were mainly aimed at the energy barrier to ring rotation in these compounds, since this barrier determines whether an axial chiral PCB or metabolite will undergo thermal racemisation under certain environmental or analytical conditions. Exhaustive semiempirical calculations have been used to relate the barrier to axial rotation to the ortho-substitution pattern and to extract simple concepts for its prediction. While semiempirical approaches are accurate enough to produce a qualitative ranking of rotational barriers of different compounds, they fail to reproduce quantitatively the rotational barrier (where known). Therefore, ab initio Hartree-Fock and density functional calculations were also carried out for unsubstituted biphenyl, 2-chloro biphenyl and 2,2′,6-trichloro biphenyl. These are in good agreement with experimental data. The calculated data have been used to derive a simple increment scheme for the quick estimation of the rotational barrier from the ortho-substitution pattern. It predicts barriers to be about 40, 80, 145 and 210 kJ/mol for 1, 2, 3 and 4 ortho chlorines, respectively. An ortho-methylthio substituent has about the same effect on the rotational barrier as chlorine, whereas an ortho-methylsulfonyl group causes a slightly higher barrier.