The thermodynamic function data, including total energy (E_T), enthalpy (H⁰), entropy (S⁰), free energy (G⁰) and constant volume molar heat capacity (C_V⁰) for dibenzo-furan and 135 polybrominated dibenzo-furans (PBDF), were predicted by fully optimized calculation and frequency analysis at the B3LYP/6-31G* level. In addition, the dependence of these thermodynamic parameters on the number and the position of bromine substitution were discussed. It was suggested that the total energy, enthalpy and free energy of PBDF vary remarkably with the positions of bromine substitution, with the order of decrease of parameter value being 1(9) position＞3(7) position＞2(8) position＞4(6) position. It was also found that the thermodynamic parameters are all increased in the case of the two bromine substitutions on the same aromatic ring, compared with that on two aromatic rings, with the increase effect order being ortho>>meta＞para. Moreover, the increment of the H⁰ and G⁰ values of PBDF congeners with both 1 and 9 positions substituted with bromine is larger than the corresponding values of that with substitution at ortho position, meta position and para position respectively. With addition of each bromine atom to PBDF, the increased values of S⁰ and C_V⁰ are about 40.1 and 16.3 J•mol^－1•K^－1 respectively. The isodesmic reactions were designed for calculating standard formation heat (H_f⁰) and standard free energy (G_f⁰) of PBDF. The relative stability of PBDF congeners was theoretically proposed in this work. The most stable and unstable isomer in different isomer groups of PBDF are similar to those for PCDF.

Key words: polybrominated dibenzo-furan, DFT method, thermodynamic property, relative stability