Density Functional Theory Studies on the Thermodynamic Properties of Polychlorinated Diphenyl Sulfides

Thermodynamic properties, including entropy (Sθ), heat capacity at constant volume (Cθv), enthalpy (Hθ), and Gibbs energy (Gθ) for 209 polychlorinated diphenyl sulfides (PCDPSs) in the ideal gas state at 298.15 K and 101.325 kPa have been computed using density functional theory (DFT) at the B3LYP/6-31G* level with the Gaussian 98 program. The standard enthalpy of formation (ΔfHθ) and the standard Gibbs energy of formation (ΔfGθ) were obtained by designing isodesmic reactions. The relations of Sθ, Cθv, ΔfHθ, and ΔfGθ with the number and position of chlorine substitution (NPCS) are discussed. It suggested that Sθ, Cθv, ΔfHθ, and ΔfGθ of PCDPSs vary greatly with NPCS. The values of heat capacity at constant pressure (Cθp) at temperatures from (200 to 1800) K for PCDPS congeners were calculated using a statistical thermodynamics calculation program based on Gaussian output files, and their temperature dependencies were obtained using the least-squares method. On the basis of the magnitude of the relative standard Gibbs energy of formation (ΔR,fGθ), the relative stability of PCDPS isomers is theoretically proposed and compared with that of the polychlorinated diphenyl ether (PCDE) isomers.