Molecular structures of 22 tetrachlorinated dibenzo-p-dioxins (TCDDs) were calculated by the B3LYP/6-311G~（**） method. The structures, total energy, standard entropy, standard enthalpy and standard free energy were obtained, in which the calculated molecular configuration of 2,3,7,8-TCDD is very close to the experimental analysis of X-ray diffraction. The results show that the total energy, the standard enthalpy and standard free energy are very well correlated with the numbers of chlorine atoms at the a positions and the parameters that represent the specific configurations of two chlorine atoms. The linear relation coefficient r is larger than 0.997. Based on the above result, a relative stability order of the total energy and standard free energy of all isomers was established. The total energy and the standard free energy of 1,3,6,8-TCDD are the lowest compared to other isomers, so it is the most stable configuration with 1,3,7,9-TCDD being the second and 1,3,7, 8-TCDD being the third in the stability of the configurations. Finally, the relative stability order was compared with the isomer pairs distribution produced in the fly ash emitted from municipal waste incinerators. Therefore, the most abundant isomer was predicted as the most stable isomer and the significance of theimodynamic control was suggested in the TCDD formation mechanism.

Key words: dioxin, DFT, THERMODYNAMICS, STABILITY, CONFIGURATION, XRD