Thermodynamic Properties of Polybrominated/chlorinated Dibenzo-p-dioxins Calculated by Density Functional Theory

Heat capacities, standard enthalpies of formation and entropies for 339 polybrominated/chlorinated dibenzo-p-dioxins (PXDDs) in the gas state were calculated using the density functional theory to construct a consistent set of thermodynamic values. The heat capacities and the entropies for PXDDs increase with increase in the substitute number of bromine and chlorine. As the degree of bromination increases, the standard enthalpies of formation and the Gibbs free energies of formation of PXDDs increase, while the degree of chlorination has only a small effect on their values compared with bromination. A much higher bromine potential than chlorine potential is needed for the formation of brominated dioxin congeners. However, the bromine-chlorine equilibrium potential for the formation of PXDDs suggested thermodynamically the possibility of their priority formation under the usual gaseous conditions where HBr and HCl are similar levels, because the bromine potential of the gas is much higher than the chlorine potential when the partial pressure of the two is similar.