DFT calculation on the thermodynamic properties of polychlorinated dibenzo-p-dioxins: Intramolecular Cl-Cl repulsion effects and their thermochemical implications
- DFT calculation on the thermodynamic properties of polychlorinated dibenzo-p-dioxins: Intramolecular Cl-Cl repulsion effects and their thermochemical implications
- Lee, JE; Choi, W; Mhin, BJ
- Date Issued
- AMER CHEMICAL SOC
- 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 (DeltaH(f)) 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 DeltaH(f) values between OCDD and the most toxic congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin, or other laterally chlorinated (2,3,7,8-substituted) toxic congeners are within 1 kcal/mol. Most DeltaH(f) values for congeners with five to eight chlorines differ by only 1-2 kcal/mol, since the decreasing electronic energies with increasing the number of chlorines in PCDDs are counterbalanced by increasing Cl-Cl repulsion energy. The intramolecular chlorine repulsion effects in PCDDs are systematically analyzed by using isodesmic reactions.
- STANDARD MOLAR ENTHALPY; CHLORINATED BENZENES; AROMATIC-COMPOUNDS; GAS-PHASE; FLY-ASH; DEHALOGENATION; COMBUSTION; PREDICTION; PATTERN; PHENOLS
- Article Type
- JOURNAL OF PHYSICAL CHEMISTRY A, vol. 107, no. 15, page. 2693 - 2699, 2003-04-17
- Files in This Item:
- There are no files associated with this item.
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.