Theoretical study on hydrophilicity and thermodynamic properties of polyfluorinated dibenzofurans

Logarithm values of octanol-water partition coefficients (log K-ow) of polyfluorinated dibenzofurans (PFDFs) were calculated based on group contributions, and their thermodynamic properties in the ideal gas state at 298.15 K and 1.013 x 10(5) Pa were calculated using a combination of quantum mechanical computations performed with the Gaussian 03 program at the B3LYP/6-311G* level. The isodesmic reactions were designed to calculate standard enthalpy of formation (Delta H-f(0)) and standard free energy of formation (Delta G(f)(0)) of PFDF congeners. By establishing relationships between these properties (including log K-ow, standard state entropy S-0, Delta H-f(0), Delta G(f)(0)) and the number and position of fluorine atom substituents (N-PFS), it was found that the fluorine substitution pattern strongly influences all of these properties of the compounds. The relation curve of log K-ow values varying with the fluorine substitution pattern presented a three-level sawtooth shape. Intramolecular repulsive forces exist between an oxygen-adjacent fluorine atom on phenyl ring and the oxygen of dibenzofuran, between ortho-substituted fluorine atoms on a phenyl ring, and between two adjacent fluorine atoms on different phenyl rings. Their repulsion energies were ascertained by comparing Delta G(f)(0) values to be approximately 14 kJ mol(-1)-1-17 kJ mol(-1), 18 kJ mol(-1)-22 kJ mol(-1) and 7 kJ mol(-1)-9 kJ mol(-1), respectively. According to the relative magnitude of their Delta G(f)(0), the relative stability order of PFDF congeners was theoretically proposed. (C) 2011 Elsevier Ltd. All rights reserved.