Studies of thermodynamic properties and relative stability of a series of polyfluorinated dibenzo-p-dioxins by density functional theory

The thermodynamic properties of 75 polyfluorinated dibenzo-p-dioxins (PFDDs) in the ideal gas state at 298.15 K and 1.013 x 105 Pa have been calculated at the B3LYP/6-311G* level using Gaussian 03 program. The isodesmic reactions were designed to calculate standard enthalpy of formation (ΔHfo) and standard free energy of formation (ΔGfo) of PFDDs congeners. The relations of these thermodynamic parameters with the number and position of fluorine atom substitution (NPFS) were discussed, and it was found that there exist high correlations between thermodynamic parameters (entropy (So), ΔHfo and ΔGfo) and NPFS. According to the relative magnitude of their ΔGfo, the relative stability order of PFDD congeners was theoretically proposed.