DFT calculation on 204 polychlorinated biphenyls: their thermodynamic function and implication of Cl substitute position

The thermodynamic function, including total energy (TE), enthalpy (H0), entropy (S0) and free energy (G0) for 204 polychlorinated biphenyls (PCBs), were predicted by fully optimized calculation at the B3LYP/6-31G* level. With design of isodemic reactions, the standard formation heat (ΔHf0), standard formation free energy (ΔGf0) of PCBs were obtained. In addition, the dependences of these thermodynamic parameters on the number and position of chlorine substitute were discussed. It was suggested that TE, H0 and G0 of PCBs vary greatly with chlorine substitute at different positions, in which the order of increase of parameter value being position 2(6)≫position 3(5)>position 4. It was also found that the data of these thermodynamic parameters all increase in the case of the two chlorine substitutes at the same aromatic ring, compared with that at two aromatic rings, the increase effect being ortho≫meta>para. With addition of each chlorine atom in PCB, the increase of S0 is about 30 J/mol K, and also the relative stability of PCB congeners at 573.15 K was proposed in this work.