AB INITIO CALCULATIONS OF STRUCTURES OF POLYCHLORINATED BIPHENYLS FOR USE IN PREDICTING PARTITIONING ONTO SOOT

The observation that soot can heavily influence the transport, bioavailability and fate of organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), has lead to research on the interactions of these two components of soils and sediments. Two purposes may be served by performing molecular simulations that parallel the experimental adsorption studies. First, molecular-level interactions organic contaminants and soot can be studied. Second, partition coefficients may be predicted in less time than required to measure them. Recent work has shown that other organic contaminants, such as PCBs, may be strongly adsorbed onto soot, but that the planarity of the structure may strongly influence the adsorption partitioning of these compounds. This research has used semi-empirical and ab initio molecular orbital/density functional theory calculations to model the structure of selected PCBs (4-chlorobiphenyl, 4,4'-dichloroBP, 2,2'.dichloroBP, 2,2',6,6' tetrachloroBP). The PCBs were selected to represent non-ortho and ortho-substituted PCBs, respectively. In addition, dibenzo-p-dioxin and dibenzofuran were also modeled. The ortho-substituted PCBs are highly non-planar as expected which corresponds to their adsorption coefficients being one order of magnitude less than the more planar non-ortho-substituted PCBs. Any pi-pi interaction that might occur between the PCBs and soot would be a distance about 3 to 4 Angstroms greater than for the planar molecules; hence, weak van der Waal’s forces are probably all that bonds these particular PCBs to soot. Interactions of these compounds with model soot surfaces are currently being calculated.