THEORETICAL CONSIDERATIONS ABOUT CHIRAL PCBS AND THEIR METHYLTHIO ANDMETHYLSULFONYL METABOLITES BEING POSSIBLY PRESENT AS STABLE ENANTIOMERS

Ortho-substituted polychlorinated biphenyls (PCBs) as well as derivati ves which contain one methylthio or methylsulfonyl group (in ortho pos ition as well) were studied by quantum chemical methods. The calculati ons were mainly aimed at the energy barrier to ring rotation in these compounds, since this barrier determines whether an axial chiral PCB o r metabolite will undergo thermal racemisation under certain environme ntal or analytical conditions. Exhaustive semiempirical calculations h ave been used to relate the barrier to axial rotation to the ortho-sub stitution pattern and to extract simple concepts for its prediction. W hile semiempirical approaches are accurate enough to produce a qualita tive ranking of rotational barriers of different compounds, they fail to reproduce quantitatively the rotational barrier (where known). Ther efore, 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 or tho-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 or tho-methylthio substituent has about the same effect on the rotational barrier as chlorine, whereas an ortho-methylsulfonyl group causes a s lightly higher barrier. (C) 1997 Elsevier Science Ltd.