Gibbs free energies of formation of pcdds: Evaluation of estimation methods and application for predicting dehalogenation pathways

Abstract

Gibbs free energies of formation for the ideal gas, liquid phase, and species in solution were estimated for 75 polychlorinated dibenzo‐p‐dioxins (PCDDs) at 298.15 K and 100,000 Pa. These values were used to calculate standard redox potentials, with H2 as the electron donor. Owing to the lack of experimental thermodynamic data for the involved chemical species, the standard‐state ideal gas enthalpies of formation were evaluated by several semiempirical methods including the Modified Neglect of Diatomic Differential Overlap (MNDO), Austin Model 1 (AM1), and MNDO–Parametric Method 3 (PM3). Three different implementations of Benson's group additivity method for estimation of ideal gas enthalpies of formation were also evaluated. As a result of the evaluation, the CHETAH program based on Benson's group additive method was chosen for ideal gas enthalpy of formation calculations. Entropies, vapor pressure, and aqueous solubility were calculated by existing methods in order to complete the thermodynamic cycle. Calculated redox potentials indicate that chlorodioxins, including 2,3,7,8‐tetrachlorodioxin, can serve as electron acceptors in anaerobic environments, at least from a thermodynamic perspective.

Photodegradation of tetra- and hexachlorodibenzo-p-dioxins

“…The differences between the findings of this study and the theoretical prediction might contribute to the differences in assumption conditions. Huang et al [27] and Lynam et al [26] used anaerobic environment condition as a basis for deducing the reductive dechlorination pathways. Meanwhile, this study involved an open system, which was not an anaerobic environment.…”

“…Huang et al[27] reported Gibbs free energy values and redox potentials for the reductive dechlorination of dioxins with hydrogen as the electron donor. The results postulated that a dechlorination pathway would follow the redox potential of the various couples, and that the couples that produced the highest energy yield would be proceeded preferentially.…”

Thermodynamic Properties of Polybrominated Dibenzo-p-dioxins and Dibenzofurans Calculated by Density Functional Theory

“…[6][7][8] The equations used for computing thermochemical data in Gaussian programs are derived from statistical thermodynamics. Two key ideas of statistical thermodynamics are the Boltzmann distribution and the partition function.…”

“…All values for the heat capacity, entropy, enthalpy and energy of formation of the 76 PBDDs increase, as the number of substituted bromines increases. For isomers of tetrabromodibenzo-p-dioxins, 1, 3,6,1,3,7,1,3,7,9-TeBDD and the most toxic compound 2,3,7,8-TeBDD are more stable than the others, and easier to form during formation process. Comparing with PBDDs, the formation enthalpies and Gibbs energies of PBDF isomers are more variable.…”

Isomer-specific investigation of PCDD/F mobility and other fate processes in deep soil cores

“…4B), where deeper core layers are primarily differentiated from surface layers based on 1,4,6,9-substituted isomers (TCDD3, PnCDD2, HxCDD1, HpCDD1). Removal of lateral chlorines has been demonstrated as a favourable theoretical dechlorination pathway for higher chlorinated PCDDs based on the Gibbs free energy of dechlorination products (Huang et al, 1996). In the soil environment, dechlorination pathways also depend on prevailing conditions and mixed peri and lateral pathways are often observed; however, preferred lateral dechlorination pathways have been demonstrated under laboratory and field conditions for both abiotic and biotic reductive environments (Barkovskii and Adriaens, 1996;Gaus et al, 2002;Uchimiya and Masunaga, 2007), and relatively long half-lives (142-1845 years) have been reported for 1,4,6,9-substituted PCDDs (compared to 2,3,7,8-substituted congeners) in the Baltic Proper (Kjeller and Rappe, 1995).…”