JCPE Journal, Vol.13, No.4, 251-254 (2001)
投 稿 論 文
Ab initio and DFT Calculations of Tetrachlorodibenzo-p-dioxins
Zun Yao Wang, †Toshio Watanabe and Osamu Kikuchi*
Department of Chemistry, University of Tsukuba, Tsukuba 305-8571, Japan
†
Department of Chemical Engineering, Yancheng Institute of Technology,
Yancheng 224003, ChinaReceived 17 August 2001; accepted 15 October 2001
HF/6-311 G* * and B3LYP/6-311 G* * calculations of tetrachlorodibenzo-p-dioxins (TCDDs) predicted that the lowest-energy isomer is not the most toxic 2,3,7,8-TCDD isomer but l,3,6,8-TCDD. This is contrary to the AM1 and PM3 predictions which are used for the elucidation of the isomer distribution of the TCDD homologue in combustion-derived samples.
Polychlorinated dibenzo-p-dioxins (PCDDs) emitted from municipal waste incinerator
display a constant pattern of the isomer distribution in various samples and conditions [1,2]. For
tetrachlorodibenzo-p-dioxins (TCDDs), 1,3,6,8-TCDD and 1,3,7,9-TCDD are most abundant
isomers in incinerator fly ash and l,3,7,8-TCDD is the third [2-4]. The abundance of the most
toxic 2,3,7,8-TCDD is less than these three isomers. The constant isomer distribution suggests
that the distribution might be controlled by thermodynamic properties of individual TCDD
isomers, such as Gibbs free energy of formation. In order to elucidate the relative abundances of
the isomers of the TCDD homologue, it is necessary to estimate the relative stability of the
isomers theoretically. The MO calculations are well suited to this purpose.
Ab initio and DFT studies have been performed on the structure [5,6], interaction [7,8] and
reactivity [9-12] of PCDDs. For the identification of the isomers, vibrational spectra were
calculated by DFT for selected TCDD isomers [13]. Very recently, Mhin et al. [14] calculated IR
spectra of all 76 PCDD congeners using the B3LYP/6-31G** method. However, they did not
give the relative energies of the isomers of each homologue.
Figure 1. Numbering of atoms in dibenzo-p-dioxin
Semi-empirical AM1 and PM3 methods are convenient for the calculation of the relative energies of the PCDD isomers, since the thermochemical parameters of molecules are obtained as the computational output of the published computer programs. Koester and Hites [15] calculated heats of formation for all TCDD isomers by the MNDO method to predict the gas chromatographic retention indexes. Unsworth and Dorans [16] calculated relative Gibbs free energy of formation by the MNDO method. The calculated equilibrium isomer composition of the TCDD homologue was found to compare well with experimental dioxin profiles of combustion sources; three most stable isomers are 1,3,7,8-, 1,3,6,8-, and 1,3,7,9-TCDD. Wehrmeier et al. [17] tried to elucidate the isomer distribution based on the AM1 calculation which predicts 2,3,7,8-TCDD to be the most stable isomer (2.5 kcal/mol more stable than 1,3,6,8-TCDD at 598K, see Table 1) in 1,3,6,8-TCDDs. Saito and Fuwa [18] used PM3 method to compare the thermodynamic parameters of PCDDs; 2,3,7,8-TCDD was calculated to be the most stable isomer in TCDDs. The present study intends to point out that the relative stability of several stable TCDD isomers obtained by ab initio and DFT calculations is contrary to that obtained by semi-empirical AM1 and PM3 calculations.
The structure and energy were calculated for 22 TCDD isomers by the HF/6-311 G* * and B3 LYP/6-311 G* * methods using the Gaussian 98 program [19]. The B3 LYP functional is quite successful for the energetic study of polychlorobenzenes [20]. Since the equilibrium geometries of TCDDs were predicted to be planar [5,6,14] and our present B3LYP/6-311 G** calculation also predicted 2,3,7,8-TCDD to be planar, the planarity was assumed in the geometry optimization of all TCDD isomers. The calculated relative energies are listed in Table 1.
The HF/6-311 G* * and B3 LYP/6-311 G* * methods gave almost the same order for the relative energies. 1,3,6,8-TCDD is the most stable and 1,3,7,9- and 1,3,6,8-TCDDs are the next most stable isomers of the TCDD homologue. This agrees well with the isomer distributions from incinerator fly ash [2-4] and suggests that the isomer distribution could be thermodynamically controlled. The AM1 and PM3 prediction of the order of stability for the lower-energy TCDD isomers, which are used for the elucidation of the observed isomer distribution, are contrary to the present ab initio and DFT prediction.
The AM1 [17] and PM3 [18] calculations predict that 2,3,7,8-TCDD is the most stable isomer. This does not match to the observed chlorination pattern in which 2,3,7,8-TCDD is not the most abundant isomer. Wehrmeier et al. [17] attributed the observed little abundance of 2,3,7,8-TCDD in combustion processes to its high reactivity. They proposed that 2,3,7,8-2,3,7,8-TCDD is destroyed faster due to its high reactivity and its abundance is less than that expected from its thermodynamic stability. It is not clear how the reactivity affects the isomer distribution in each experimental conditions. However, it should be in mind that the order of the isomer stability obtained by AM1 and PM3 calculations differs from the order of the relative energies obtained by the HF/6-311 G* * and B3LYP/6-311 G* * calculations.
Table 1. Calculated relative energies (kcal/mol) of tetrachlorodibenzo-p-dioxins
References
