DFT Calculation on the Thermodynamic Properties of Polychlorinated Dibenzo-p-dioxins: Intramolecular Cl−Cl Repulsion Effects and Their Thermochemical Implications

Polychlorinated dibenzo-p-dioxins (PCDDs) are one of the most intensively studied chemical pollutants. However, the absence of reliable thermodynamic data on PCDDs seriously limits quantitative understanding of their formation and distribution at combustion sources. By carrying out a DFT calculation (B3LYP/6-31G**) for thermodynamic properties of 75 PCDD congeners, we find that previous thermodynamic estimations on the gas-phase enthalpies of formation (ΔHf) and Gibbs free energy of PCDDs were seriously misestimated, particularly for highly chlorinated congeners, largely as a result of underestimating the intramolecular Cl−Cl repulsion energy. The disagreement between the DFT calculation and other methods becomes progressively larger with increasing chlorine substitution. Octachlorodibenzo-p-dioxin (OCDD) that has been considered as the most thermodynamically stable congener in previous calculations turns out to be much less stable. The differences in calculated ΔHf values between OCDD and the most toxic ...