Fully optimized calculation and frequency analysis of 209 polychlorinated diphenyl sulfide (PCDS) compounds were carried out at the B3LYP/6-31G* level and their structural and thermodynamic parameters were obtained. The relation of these parameters with the number and relative positions of Cl atom substitution (N_PCS) was studied, and it was found that there exists high correlation between the molecular mean polarizability (α), enthalpy (H $), free energy (G $), heat capacity at constant volume ( ) or entropy (S $) and N_PCS (correlative coefficients r²＞0.988). It was also found that the molecular volume (V_m), the energy of highest occupied molecular orbital (E_HOMO) and the energy of lowest unoccupied molecular orbital (E_LUMO) had good correlation with N_PCS and the correlative coefficients r² were 0.949, 0.894 and 0.915 respectively. The isodesmic reactions were designed to calculate standard enthalpy of formation (Δ_fH $) and standard free energy of formation (Δ_fG $) of PCDS congeners. The order of relative stability of these PCDS congeners was theoretically proposed based on the relative magnitude of their Δ_fG .

Key words: polychlorinated diphenyl sulfide, DFT method, position of Cl atom substitution method, structural parameter, thermodynamic property, relative stability