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

Abstract

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 (∆H 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 ∆H f values between OCDD and the most toxic congener, 2,3,7,8-tetrachlorodibenzop-dioxin, or other laterally chlorinated (2,3,7,8-substituted) toxic congeners are within 1 kcal/mol. Most ∆H 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.

DFT Calculation on 76 Polychlorophenazines: Their Thermodynamic Function and Implication of Cl Substituted Position

“…Lee et al [18] used the isodesmic reaction to calculate the Δ f H and Δ f G of PCDDs and obtained results consistent to experimental results. So it seems reasonable to expect that estimates of the thermodynamic properties of PCPZs, from DFT calculations based on isodesmic reactions, will be similarly accurate.…”

Chlorination pattern effect on thermodynamic parameters and environmental degradability for C 10 -SCCPs: Quantum chemical calculation based on virtual combinational library

“…The effect of chlorination pattern on the environmental degradability of C 10 -SCCPs was further analyzed by evaluating its influence on direct photolysis, oxidation by hydroxyl radical as well as other transformation processes of C 10 -SCCPs involving nucleophilic substitution reactions. Frontier molecular orbitals have been employed to depict the reactivity of pollutants extensively, among which the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play a fundamental role in the characterization of the nucleophilicity and electrophilicity of a molecule, respectively (Lee et al, 2003). In general, E HOMO directly correlates with the susceptibility of a molecule toward the attack by electrophilic substances, while the nucleophilic substitution reactions easily occur for the molecules with low E LUMO .…”

Assessment of Gaussian-3X theory for chlorinated organic molecules. Enthalpies of formation of chlorobenzenes and predictions for polychlorinated aromatic compounds

“…As already noted, in the recent years the enthalpies of formation of different polychlorinated aromatic compounds were predicted from B3LYP calculations using isodesmic reactions [8][9][10][11][12][13][14][15]. To estimate the f H • 298 values of PCDDs, the following reactions may be considered as the most appropriate:…”

“…The thermodynamic properties of chlorinated aromatic species are important in the kinetic modeling their formation during the processes of pyrolysis and oxidation. A large number of studies on the estimation of thermochemical properties of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans, naphthalenes, and biphenyls has been published last years (see [8][9][10][11][12][13][14][15], and references cited therein). In these works, the enthalpies of formation were calculated using the B3LYP total energies and isodesmic reactions.…”