Structures, potential energy curves, and torsional barrier heights for selected polychlorinated biphenyls: A density functional theory study

Density functional theory studies were performed to obtain the structures, rotational barriers, and potential energy curves of six selected polychlorinated biphenyls (PCBs): 3,3',4,4'- and 2,2',5,5'-tetrachlorobiphenyl; 3,3',4,4',5-, 2,2',4,5,5'-, and 2,3',4,4',5-pentachlorobiphenyl; and 3,3',4,4',5,5'-hexachlorobiphenyl. Becke's three-parameter hybrid density functional, B3LYP, combined with 6-31G(d), 6-311G(d,p), and 6-311+G-(2d,2p) basis sets was utilized for this purpose. For the selected PCBs, we present optimized geometries at the B3LYP/6-311 +G(2d,2p) level of theory; torsional barriers at 0degrees and 90degrees at the B3LYP/6-31G(d), B3LYP/ 6-311G(d,p), and B3LYP/6-311+G(2d,2p) levels of theory; and potential energy curves (relative energy vs torsional angle) at the B3LYP/6-311G(d,p) level of theory. The geometries, torsional barriers, and potential energy curves of the non-ortho-chlorinated PCBs mimicked those of their parent biphenyl, whereas the remaining selected PCBs showed different behaviors. The syn-like structures of the 2,2',5,5'- and 2,2',4,5,5'-PCBs were of particular interest. However, the energy difference between the near-syn-like and near-anti-like structures was very small. Both of these PCBs have large barriers near planarity.