Heat capacities and entropies for 76 polybrominated dibenzo-p-dioxins (PBDDs) in the gas state at 298.15 K and 101.325 kPa have been computed using the density functional theory (B3LYP/6-31G(d)) with Gaussian 98. Based on the output data of Gaussian, three methods were employed to calculate enthalpies and Gibbs energies of formation of the 76 PBDDs in the gaseous state at 298.15 K and 101.325 kPa. To assess the three methods, thermodynamic properties of 16 compounds were first calculated by B3LYP/6-31G(d) and compared with reference values. For predicting the enthalpies of formation of the reference compounds, method 2 has the smallest average absolute deviation from the experimental data. 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, 1,3,6,8-TeBDD, 1,3,7,8-TeBDD, 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 the formation process.

An apparatus for vapor pressure measurement with a very small cell by the mass-loss Knudsen effusion technique was tested with solid benzoic acid and anthracene. The vapor pressure and enthalpy of sublimation results of the two reference compounds were in good agreement with accepted literature data. The vapor pressures at different temperatures of 17 polychlorinated dibenzo-p-dioxins (including dibenzo-p-dioxin) and five polychlorinated dibenzofurans (including dibenzofuran) were measured with the apparatus, and the enthalpies of sublimation of the 22 dioxins and furans were derived from the temperature dependence of vapor pressure. The results were systematically compared with the literature data.

A new apparatus with Knudsen effusion method especially designed for the vapour pressure measurements of dioxin congeners and other POPs is described. Crystalline benzoic acid and anthracene were first used to test the new designed apparatus. The vapour pressure and enthalpy results of the two reference compounds were found in good agreement with accepted literature data. The vapour pressure and enthalpy results of crystalline DD, DF, OCDD and OCDF determined with the new apparatus are presented, and compared with the literature data measured with other methods.

Heat capacities and entropies for 76 polybrominated dibenzo-p-dioxins (PBDDs) and 136 polybrominated dibenzofurans (PBDFs) in the gas state have been computed using the density functional theory. Based on the output data of Gaussian, three methods were employed to calculate enthalpies and Gibbs energies of formation of PBDDs and PBDFs in the gaseous state at 298.15 K and 101.325 kPa. To assess the three methods, thermodynamic properties of 16 brominated arenes compounds were first calculated and compared with experimental values. Among the three methods used, method 2 has the smallest average absolute deviation from the experimental data. 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. The formation enthalpies and Gibbs energies of isomers which have bromine substitutions in 1 and 9 positions are much higher than those of the others.

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