Gas-phase thermodynamic properties (enthalpy of formation, entropy, and heat capacity) are estimated for dibenzo-p-dioxin (DD), dibenzofuran (DF), 75 possible isomers of polychlorinated dibenzo-p-dioxins (PCDDs), and 135 of polychlorinated dibenzofurans (PCDFs). To estimate the enthalpy of formation values, the difference method is used that is completely consistent with the group additivity approach. Entropies and heat capacities are calculated by statistical thermodynamics using evaluated structural parameters and vibrational frequencies. Normal coordinate analysis is carried out for all isomers of PCDDs and PCDFs by transferring the force constants from polycyclic aromatic hydrocarbons, dibenzofuran, and chlorinated benzenes. The group additivity scheme with 10 parameters is developed to approximate calculated thermodynamic properties. Results obtained in this work are compared with published experimental and theoretical data.

The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X(MP2), and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta f H298 degree values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)3, P(C2H5)3, P(OCH3)3, n-C4H9OPCl2, [(CH3)2N]2PCl, (C2H5)2NPCl2, and [(CH3)2N]2PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)3 and P(n-C4H9)3. From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta f H298 degree values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.

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The enthalpies of formation of chlorinated methanes, ethanes, ethylenes, phenols, and benzenes have been calculated at the G3X level of theory using the atomization energy procedure and the method of isodesmic reactions. By comparing the most reliable experimental data on chlorinated hydrocarbons recommended by Manion [Manion JA (2002) J Phys Chem Ref Data 31:123] with the G3X results, the accuracy of theoretical enthalpies of formation is estimated as ranging from ± 4 to ± 10 kJ/mol. Only for hexachloroethane, the difference between the experimental value and G3X result was outside this range and the experimental enthalpy of formation of hexachloroethane was called into question by theory. The G3X enthalpies of formation of all chlorobenzenes agree well with experimental data which were partly reanalyzed using recent experimental data on enthalpies of sublimation. Based on the G3X results, a set of self-consistent experimental data for chlorobenzenes is recommended. The enthalpies of formation of some polychlorinated dibenzo-p-dioxins were estimated using improved enthalpies of formation for chlorobenzenes. The possible inaccuracy of previously estimated values for polychlorinated aromatic compounds is discussed.

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