Determination of the rotational energy barriers of atropisomeric polychlorinated biphenyls

The rotational energy barriers were determined for twelve of the nineteen environmentally stable atropisomeric polychlorinated biphenyls (PCBs), viz. PCB 84, 131, 132, 135, 136, 144, 149, 174, 175, 176, 183, and 196, by thermal racemization of enantiomerically pure PCBs. The rate of racemization was primarily determined using off-line gas chromatography (GC) or high-performance liquid chromatography (HPLC), with permethylated cyclodextrin (PMCD) as the chiral selector. GC was used for PCB 84, 132, 136, 149, 174, and 176, while PCB 131, 175, and 196 were analyzed using HPLC. The remaining PCBs, i.e. PCB 144 and 183, were separated by HPLC using a Chiralpak OP(+) polymethacrylate column. Gibbs free energy of activation (Δ‡G) was 176.6 to 184.8 kJ/ mole for tri-ortho PCBs. For tetra-ortho PCBs the Δ‡G was estimated to be ∼246 kJ/mole. A buttressing effect of 6.4 kJ/mole was observed for tri-ortho PCBs that have one buttressing chlorine.