TOXICITY of polychlorinated biphenyls (PCBs) has seen an upsurge of interest in recent years. These compounds exhibit toxicity similar to that of polychlorinated dibenzo-p-dioxin (PCDD). This information on PCB has prompted several investigators to understand the toxic nature of PCBs and their interaction with cellular components.

The origin of toxicity of PCDDs has been attributed to the electron accepting nature in charge transfer complex with a receptor in living cells. Hence electron affinity of PCBs is used as an important quantity in understanding their toxic effects. Recently,  Arulmozhiraja et al. have made an analysis on structure, potential energy and torsional barrier heights for selected polychlorinated biphenyls. Rotational energy barrier, electron affinity and planarity of various PCBs have been calculated in that study to rationalize the non-toxic nature of ortho-substituted PCBs. Rotational energy barriers of biphenyls (BP) and substituted biphenyls have been calculated using B3LYP/6-311+G* calculations by Grein.

There are similar calculations on torsional barrier  of BP and PCB using various theoretical calculations ranging from semi-empirical AM1 to conventional Hartree–Fock  methods. It is evident from the calculations that the toxicity arises mainly from the electron affinity and inherent nature of the planar geometry of the biphenyls and substituted biphenyls. It is well known in gas phase that BP is twisted (torsional angle between two phenyl rings) with twist angle of about 45°. This twist in BP is usually explained as arising from competition between the repulsion of the ortho hydrogens favouring 90° twists (torsional angle  f) and the electron delocalization effect preferring a coplanar arrangement. In chlorinated BPs, this balance in interactions is still perturbed by the chlorine atoms, which influences  the geometrical parameters of BPs specifically the torsional angle between the phenyl rings.It is evident from the previous theoretical studies that torsional angle is not influenced by the chlorine substituents at the para and meta positions.

 However, torsional angle between two phenyl rings with ortho substitution is nearly 90°. In real life systems, PCBs are known to interact with the cellular components and hence addition and removal of electron during the formation of the complex are very significant events. The electron acceptance as well as electron removal to PCBs lead to changes in the torsional angle f of PCBs and hence their geometry. Popular qualitative chemical concepts like electronegativity and hardness have been widely used in understanding various aspects of chemical reactivity.

Rigorous theoretical basis for these concepts has been provided by density functional theory. These reactivity indices are better appreciated in terms of the associated electronic structure principles such as electronegativity equalization principle, hard–soft acid base (HASB) principle, maximum hardness principle (MHP), minimum polarizability principle (MPP), etc. Local reactivity descriptors like density, Fukui function, local softness, etc. have been used successfully in the studies of site selectivity in a molecule. It is reported in the earlier study that the rotational freedom of PCBs allows it to orient with any torsional angle in the protein field and provides the pathway for easy interaction with receptors in living cells and hence their toxicity. In this investigation, an attempt has been made to examine how various chemical reactivity and selectivity indices and their associated electronic structure principles manifest themselves when  PCBs rotate in the realistic environment so that a proper descriptor  can be selected to define toxicity of various compounds.