Density functional theory studies on the molecular structures and thermodynamic properties of polychlorinated anthracenes

Geometric molecular structures of 287 polychlorinated anthracenes (PCANs) were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level and their thermodynamic properties in the ideal gas state (heat capacity at constant volume (Cθv), entropy (Sθ), enthalpy (Hθ), Gibbs energy (Gθ), standard enthalpy of formation (ΔfHθ) and standard Gibbs energy of formation (ΔfGθ)) were computed. The relations of Cθv, Sθ, ΔfHθ and ΔfGθ with the number and position of chlorine atoms were also explored. According to the magnitude of the relative standard Gibbs energy of formation (Δr,fGθ), the relative stability of PCAN congeners was theoretically proposed. It was found that some PCAN isomers have a planar configuration while others adopt a nonplanar configuration. Both the values of Cθv and Sθ increase with successive chlorination while the values of ΔfHθ and ΔfGθ of the most stable PCAN isomers decrease initially and then increase with increasing the degree of chlorination. The values of ΔfHθ and ΔfGθ of PCAN congeners with the same number of chlorine atoms show a strong dependence on the position of chlorine substitution. The relative thermodynamic stability of PCAN isomers is determined mainly by relative magnitude of intramolecular delocalized π bond and Cl-Cl nuclear repulsive interaction.