Theoretical study on the molecular structures and thermodynamic properties of polychlorinated pyrenes

Geometric molecular structures of 287 polychlorinated pyrenes (PCPY) were 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 standard heat capacity at constant volume ( C v ∘ ) , standard entropy (S∘), standard enthalpy of formation (ΔfH∘) and standard Gibbs free energy of formation (ΔfG∘), were computed. The relations of C v ∘ , S ∘ , Δ f H ∘ and Δ f G ∘ with the number and position of chlorine atoms have also been explored, from which the relative stability of PCPY congeners was theoretically proposed according to the magnitude of the relative standard Gibbs free energy of formation (Δr,fG∘). It was found that some of PCPY isomers are in a nonplanar configuration. With increasing the number of chlorine atoms, the values of ΔfH∘ and ΔfG∘ of the most stable PCPY isomers decrease initially and then increase quickly. The values of ΔfH∘ and ΔfG∘ of PCPY congeners with the same number of chlorine atoms show a strong dependence on the positions of chlorine atoms. The relative thermodynamic stability of PCPY isomers is determined mainly by intramolecular adjacent Cl⋯Cl nuclear repulsive interaction between Cl atoms at two different six-membered rings.