Theoretical studies on the structure and thermodynamic properties of 2,3,7,8 - tetrachlorinated dibenzo - p - dioxin

Density functional theory (DFT) and ab initio methods were used to optimize the molecular geometry of 2,3,7,8 - TCDD at B3LYP/6 - 31G, B3LYP/6 -31G *, B3LYP/6 - 311G * and MP2/6 - 31G * levels, respectively. Harmonic vibrational spectrums have been performed at B3LYP/6-31G * and B3LYP/6 - 311G * levels. The standard thermodynamic parameters such as heats of formation, standard entropy, standard enthalpy, heat capacity at various temperatures have been evaluated using the scaled B3LYP/6 - 31G * and B3LYP/6 - 311G * frequencies. Semi - empirical molecular orbital method with the PM3 Hamiltonian at restricted Hartree-Fock level (RHF) was also performed to compare the geometrical optimization and thermodynamic properties with those methods mentioned above. The results show that: (1) 2,3,7,8-tetrachlorinated dibenzo-p-dioxin(TCDD) is a planar molecule, optimal geometries obtained by B3LYP/6 - 31G, B3LYP/6 - 31G *, B3LYP/6 - 311G * and MP 2/6 - 31G * are almost the same, the largest difference in bond length between them is 7.9 pm, the largest difference in bond angle is 0.55°, and the largest difference in rotational constant is 0.03 GHz; (2) There are 60 vibrational frequencies for this molecule, of which there are 10 strongest intensities of vibration bands. The computed frequency scaled by 0.9628 for B3LYP/6 - 31G * and scaled by 0.9711 for B3LYP/6 - 311G * have similar frequencies and intensities of vibration bands, both are in good accordance with experimental values. (3) The thermodynamic parameters such as standard entropy, standard enthalpy, heat capacity, heats of formation at temperatures from 298 ∼ 1500K have been calculated, All the thermodynamic parameters increase with the increment of temperature, and the difference between B3LYP/6 - 31G * and B3LYP/6 -311G * is small, PM3 method also predicts good results, and in good accordance with other's works and experimental values. (4) The optimal geometry and thermodynamic parameters of 2,3,7,8 - tetrachlorinated dibenzo - p - dioxin (TCDD) obtained in this paper would be fundamental databases for further research on generation, decomposition and kinetics study of dioxins.