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Environment-related properties of polyhydroxylated dibenzo-p-dioxins

Shi, Jia-Qi, Qu, Rui-juan, Flamm, Alison, Liu, Hong-Xia, Xu, Ying, Wang, Zun-Yao
The Science of the total environment 2012 v.414 pp. 404-416
benzene, energy, enthalpy, entropy, ethers, hydrogen bonding, ionization, models, octanol-water partition coefficients, oxygen, polychlorinated dibenzodioxins, synthetic products
Polyhydroxylated dibenzo-p-dioxins (PHODDs) are important metabolic and synthetic products of polychlorinated dibenzo-p-dioxins (PCDDs). Two types of hydrogen bonds exist in PHODD molecules: one between a hydroxyl group (HO) and an oxygen atom of the ether bond, and the other between two ortho hydroxyls of a benzene ring. By fully optimized calculation with density functional theory (DFT), their bond energies were ascertained to be approximately 9–14kJ/mol and 15–19kJ/mol respectively by the comparison of standard Gibbs energy of formation (ΔfGᶿ) between different molecules, which was experimentally verified. The two types of hydrogen bonds affect the hydrophilicity and stability of the molecules. The torsional potential of hydroxyls and the orientation making the congener most stable were obtained. The octanol-water partition coefficients (logKₒws) were calculated based on the group contribution method, and the standard state entropy (Sᶿ), standard enthalpy (ΔfHᶿ) of formation and ΔfGᶿ were obtained from the combination of DFT calculation and isodesmic reaction for the stable PHODD congeners. The number and position of hydroxyl substitution (NPHOS) were employed as descriptors to establish quantitative structure–property relationship (QSPR) models. Although the hydrophilicity of PHODDs increases with the number of hydroxyl groups, it is impaired by the intramolecular hydrogen bonds. The logKₒws of PHODDs are much smaller than those of PCDDs, and the variation trend with the number of substituents is different. In addition, the relative stability order of PHODD congeners was theoretically proposed, which is quite different from that of PCDDs. Considering the ionization in water, first-order ionization constants of PHODDs were calculated according to the results of SMD method of Self-Consistent Reaction Field Theory (SCRF), and they were influenced by the hydrogen bonds.