MOLECULAR-ORBITAL CALCULATIONS TO DESCRIBE MICROBIAL REDUCTIVE DECHLORINATION OF POLYCHLORINATED DIOXINS

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 our for a range of chlorin ated dioxin molecules to obtain molecular descriptors: such as HOMO-LU MO gaps (HOMO = highest occupied molecular orbital, LUMO = lowest unoc cupied molecular orbital) and partial atomic charges. The HOMO-LUMO ga p is an indicator of stability in a molecule: the larger the gap the g reater the stability of the molecule toward further reaction. These ca lculations indicate that with increasing extent of chlorination, the g ap decreases. The observed charge pattern shows that carbon atoms in t he peri (1.1,6.9) ring positions have a partial negative charge while their in thr lateral (2,3,7,8) position have a partial positive or sma ll partial negative charge. The descriptors, from the more precise DFT method, were used to rationalize experimental observations of dechlor ination of dioxins. Reductive dechlorination pathways from two differe nt experimental studies were examined using partial charges and estima ted Gibbs free energy of dechlorination. In both experimental studies, highly thermodynamically favorable and less thermodynamically favorab le pathways were observed. For a given chlorinated dioxin, when more t han one degradation pathway was possible, dechlorination in the most t hermodynamically favored pathway occurred at the most positively charg ed carbon atom in the ring, which was usually a lateral carbon atom. T hese results are discussed in light of a possible mechanism for reduct ive dechlorination.