Theoretical Indicators for Proposing Reductive Dechlorination Pathways of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans

Density functional theory (DFT) calculations were carried out at the B3LYP/6-31G(d) level for all polychlorinateddibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Mulliken atomic charge on chlorine atom (Q_Cl(n)) and molecular total energy (E_T) were adopted as the theoretical indicators of the dechlorination reaction activity. The hypothesis was that the preferred reaction pathways would either yield dominant daughter products having the lowest ET, or occur in the substituted chlorine atom having the largest Q_Cl(n). The predicted pathways were validated with literature data on the biotic dechlorination of 1,2,3,4-TCDD and the biotic reductive dechlorination pathways for 1,2,3,4-TCDD, 1,2,3,7,8-PeCDD,1,2,3,4,7,8-HxCDF. Results showed the a biotic reactions with zero-valent zinc and biotic transformations by Dehalococcoidessp. strain CBDB1 followed the pathways eliminating chlorine atom having the largest Q_Cl(n), but the biotic transformations by Dehalococcoides ethenogenes strain 195 followed the pathways that favored the lowest E_T products.