Gas phase thermodynamic properties of PBDEs, PBBs, PBPs, HBCD and TBBPA predicted using DFT method

The gas phase thermodynamic properties of 209 polybrominated diphenyl ethers (PBDEs), 209 polybrominated biphenyls (PBBs), 19 polybrominated phenols (PBPs), hexabromocyclododecane (HBCD) and tetrabromobisphenol A (TBBPA), including standard state entropy (S^{{ring operator}}), heat capacity (C_p), enthalpy (Δ H_f^{{ring operator}}) and Gibbs free energy of formation (Δ G_f^{{ring operator}}), were predicted using a combination of quantum mechanical computations performed using the Gaussian 03 program at the B3LYP/6-31G(d) level, and thermodynamic relationships derived from the law of energy conservation. The obtained results showed a strong influence of the bromine substitution pattern on the thermodynamic properties of the compounds. For instance, the thermodynamic stability of all congeners decreases with increasing number of Br atoms, especially if substituted at ortho < meta < para positions. Taking into account differences in the Gibbs free energy of formation, the most and least stable congeners in each isomer group were determined. Application of the obtained results in the thermodynamic analysis of the reported experimental stabilities of the compounds afforded the following conclusions: (i) the order of decreasing stability of the PBDEs calculated in this study is in good agreement with the increasing rate of their photochemical degradation; (ii) the formations of lower brominated polybrominated dibenzofurans (PBDFs) instead of adequate congeners of PBDEs during the photolytic debromination of decabromodiphenyl ether (DeBDE) seemed to be thermodynamically favored; (iii) the debromination of PBDEs does not follow just one thermodynamic path, but is rather multidirectional, leading to a mixture of several congeners, the concentration of which is related to their thermodynamic stability.