Computational study on the relative stability and formation distribution of 76 polychlorinated naphthalene by density functional theory

The thermodynamic properties of 76 polychlorinated naphthalenes (PCNs), including total energy (TE), enthalpy (H0), entropy (S0), free energy (G0), zero-point vibrational energy (ZPE), constant volume molar heat capacity (CV0) and thermal energy correction (Eth), were predicted by fully optimized calculation at the B3LYP/6-31G* level. The standard heat energy of formation (ΔHf0) and standard free energy of formation (ΔGf0) of PCN congeners were obtained by designing isodemic reactions. In addition, the dependences of these thermodynamic parameters on the number and the position of chlorine substitutions were discussed, in which an obvious increase of TE, H0, G0, ΔHf0 and ΔGf0 occur once chlorine substitutions are simultaneously at positions 1 and 8 (or positions 4 and 5). And with increasing number of chlorine atoms in PCN molecule, the values of S0 and CV0 increase and Eth and ZPE decrease. Based on the magnitude of the relative free energy (RΔGf0), the relative stability order for PCN isomers of each congener was theoretically determined. The most theoretically stable isomer was found in high concentration observed from partial samples in the environment, which is basically consistent with the predicted formation distribution of PCNs in this work. Compared with the four semiempirical methods, AM1, MINDO/3, MNDO and PM3, the ΔHf0 magnitude of PCNs obtained from B3LYP/6-31G* is the highest, and the differences resulted from these methods become greater with increasing number of chlorine substitute.