Prediction of gas phase thermodynamic properties of polychlorinated dibenzo-furans by DFT

The optimized calculation of dibenzo-furan (DF) and 135 polychlorinated dibenzo-furans (PCDFs) was systematically carried out at the B3LYP/6-31G* level, and their total energy (E), enthalpy (H0), entropy (S0) and free energy (G0) were all obtained. Based on observation of the dependences of the thermodynamic parameters on the number and the position of chlorine substitutions, it was suggested that PCDFs with chlorines at 1 and 9 positions synchronously are less stable than at ortho positions. By designing isodemic reactions, standard heat of formation (ΔHf0) and free energy (ΔGf0) were calculated, from which the relative stability order of PCDFs was determined. Based on comparison of the stability order with the observed percentage of hexa-chlorinated dibenzo-furans (Hexa-CDFs) and hepta-chlorinated dibenzo-furans (Hepta-CDFs) in the ash, fly ash and soil from incinerator, the stability order of the former is basically in consistency with that of the latter. The thermodynamic parameters of PCDFs achieved in the present study thus possess higher precision than those obtained by semi-empirical methods in the past researches, and it is therefore of significance to the study of formation mechanism of dioxins.