Geometric molecular structures of 527 polychlorinated phenanthrenes (PCPhen) have been optimized using density functional theory (DFT) at the B3LYP/6-311G(d, p) level and their thermodynamic properties in the ideal gas state, such as heat capacity at constant volume ( ), entropy (S$), standard enthalpy of formation (ΔfH$) and standard Gibbs free energy of formation (ΔfG$), have been computed. The relations of , S$, ΔfH$ and ΔfG$ with the number and position of chlorine atoms have also been explored, from which the relative stability of PCPhen congeners was theoretically proposed according to the magnitude of the relative standard Gibbs free energy of formation (Δr,fG$). It was found that most of PCPhen isomers are in a nonplanar configuration, with three kinds of intramolecular weak interactions, i.e., H…H, C—H…Cl and Cl…Cl interactions. With increasing the number of chlorine atoms, the values of ΔfH$ and ΔfG$ of the most stable PCPhen isomers decrease initially and then increase quickly. The values of ΔfH$ and ΔfG$ of PCPhen congeners with the same number of chlorine atoms show a strong dependence on the positions of chlorine atoms. The relative thermodynamic stability of PCPhen isomers is determined mainly by relative magnitude of intramolecularly delocalized p bond and Cl…Cl nuclear repulsive interaction.

Key words: polychlorinated phenanthrene (PCPhen), molecular structure, thermodynamic property, relative stability, density functional theory (DFT)