The thermodynamic function data, including total energy E_T, enthalpy H°, entropy S°, free energy G°, zero-point vibration energy E_ZPV, constant volume molar heat capacity C_V° and thermal energy correction E_th for 218 polychlorinated biphenyls (PCB), were predicted by fully optimized calculation at the B3LYP/6-31G* level. In addition, the dependence of these thermodynamic parameters on the number and the position of chlorine substitute was discussed. It was suggested that the total energy, enthalpy and free energy of PCB vary remarkably with the positions of chlorine substitutes, with the order of increase of parameter value being position 2(6)>>3(5)＞4. It was also found that the thermodynamic parameters were all increased in the case of the two chlorine substitutes on the same aromatic ring, compared with that on two aromatic rings, with the increase effect order being ortho>>meta＞para. With addition of each chlorine atom to PCB, the increased values of S° and C_V° were about 30 and 16 J•(mol•K)^－1, the decreased ones of E_th and E_ZPV about 22.2 and 25.5 kJ•mol^－1, respectively. Also, the relative stability of PCB congeners was theoretically proposed in this work.

Key words: dioxin, polychlorinated biphenyl, DFT method, thermodynamic property, relative stability