Abstract
Ab initio restricted Hartree Fock and density functional theory (DFT), as well as semiempirical Austin model 1 and parametrization method 3 molecular orbital calculations were carried out for a range of chlorinated dioxin molecules to obtain molecular descriptors such as HOMOโLUMO gaps (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) and partial atomic charges. The HOMOโLUMO gap is an indicator of stability in a molecule: the larger the gap the greater the stability of the molecule toward further reaction. These calculations indicate that with increasing extent of chlorination, the gap decreases. The observed charge pattern shows that carbon atoms in the peri (1,4,6,9) ring positions have a partial negative charge while those in the lateral (2,3,7,8) position have a partial positive or small partial negative charge. The descriptors, from the more precise DFT method, were used to rationalize experimental observations of dechlorination of dioxins. Reductive dechlorination pathways from two different experimental studies were examined using partial charges and estimated Gibbs free energy of dechlorination. In both experimental studies, highly thermodynamically favorable and less thermodynamically favorable pathways were observed. For a given chlorinated dioxin, when more than one degradation pathway was possible, dechlorination in the most thermodynamically favored pathway occurred at the most positively charged carbon atom in the ring, which was usually a lateral carbon atom. These results are discussed in light of a possible mechanism for reductive dechlorination.