Density Functional Theory Studies On The Static Structures And Water-exchange Reactions Of Aluminum-salicylate Complexes

Aluminum is the most abundant metal in the earth’s crust. Aqueous reactions between aluminum and carboxylic acids are important in environmental chemistry for a number of reasons. Complexation of Al3+by natural organic matter(NOM) has been shown to dramatically reduce the toxicity of the metals compared to an equivalent concentration of free ions; Meanwhile, the formation of A13+-NOM complexes can also affect the transport of aluminum in environment. Furthermore, NOM adsorbed onto aluminum minerals can control the surface electrostatic properties and sorption characteristics as well as mineral solubility and mobility, thereby affecting mineral weathering and diagenetic process. Salicylic acid is a common low-molecular-weight organic compound presented in natural environment, which is often utilized as a simple model of the complicated high-molecular weight humic acid in order to study the influence of natural organic matters on the mineral dissolution and Al toxicity. However, due to the limitation of experimental techniques, there still exits the disagreement in the structures of Al(Ⅲ)-salicylate complexes. So far, the study on the effects of organic ligands on the water-exchange reactions of aluminum species is still at a preliminary stage. The influences of the bonded functional groups, ring size, the number of rings as well as the functional groups on the chelate rings on the process of the whole water-exchange reactions have not been revealed and explained. All these are the key points in the kinetic reactions of aluminum species in natural water environment. In this dissertation, the static structures and water-exchange reactions of Al(Ⅲ)-salicylate complexes are systematically studied from the following three aspects:1. Introduction. The study of aqueous aluminum species in environment is of great environmental significance. The investigation for the effects of various organic ligands on the structures and water-exchange reactions of different aluminum species in aqueous solutions is helpful to understand the formation and transformation of mineral as well as the dissolution, transfer and transformation of the soluble aluminum in earth surface. In this chapter, the significance of aqueous aluminum-carboxylate complexes in environment has been described, the progresses for the experimental and theoretical studies on the aluminum-salicylate complexes are reviewed, and the water-exchange of various metal ions and the environmental importance of aluminum-carboxylate complexes are summarized.2. Theoretical investigation of the thermodynamic static structures and kinetic water-exchange reactions of aqueous Al(Ⅲ)-salicylate complexes. Density functional theory (DFT) calculations are performed on the structures and water-exchange reactions of aqueous Al(Ⅲ)-salicylate complexes. Based on the four models (gas phase(GP), polarizable continuum model(GP-PCM) to estimate the bulk solvent effect, supermolecule model(GP-SM) to consider the explicit solvent effect and supermolecule-polarizable continuum model (GP-SM-PCM) to take the both solvent effects into consideration), we have systematically conducted the research work from following two aspects:(1) The microscopic properties of aqueous Al(Ⅲ)-salicylate complexes with various structures (including the possible1:1mono-and bidentate complexes, cis and trans isomers of1:2bidentate complexes as well as1:3bidentate complexes) are optimized and studied at the level of B3LYP/6-311+G(d,p);(2)27Al NMR and13C NMR chemical shifts are calculated using GIAO method at the HF/6-311+G(d,p) level, the calculation results show that the achieved values with SM-PCM models are in good agreement with the available experimental data in literatures, indicating the models we adopted are appropriate for Al(Ⅲ)-salicylate complexes;(3) The water-exchange reactions on1:1mono-and bidentate Al(Ⅲ)-salicylate complexes are simulated with supermolecule models at the same computational level of B3LYP/6-311+G(d,p). The predicted the logarithms of water-exchange rate constant(logke) of the1:1bidentate complex with logkex-dAl-OH2correlation is4.0, which is in consistent with the experimental value of3.7, whereas the calculated values of logkex of1:1monodentate complexes fall in the range of1.3～1.9. By effectively combining the research of the thermodynamic static structures with the simulation of the kinetic water-exchange reactions, this work promotes our further understanding of the configuration and formation mechanism of Al(Ⅲ)-salicylate complexes.3. Theoretical investigation of the thermodynamic structures and kinetic water-exchange reactions of typtical salicylate-Al(III) dimers. Density functional theory (DFT) calculations are performed on the structures and water-exchange reactions of the typical dimers of Al(III)-salicylate complexes. Based on the four models (gas phase(GP), polarizable continuum model(GP-PCM) to estimate the bulk solvent effect, supermolecule model(GP-SM) to consider the explicit solvent effect and supermolecule-polarizable continuum model (GP-SM-PCM) to take the both solvent effects into consideration), we systematically conduct the research work from following two aspects:(1) The microscopic properties of Al2(HSal)(OH)2(H2O)63+and Al2(HSal)(OH)4(H2O)4+are optimized and studied at the level of B3LYP/6-311+G(d,p);(2) The water-exchange reactions on Al2(HSal)(OH)2(H2O)63+and Al2(HSal)(OH)4(H2O)4+are simulated with supermolecule models at the same computational level of B3LYP/6-311+G(d,p). The predicted water-exchange rate constants (logkex) of Al2(HSal)(OH)2(H2O)63+and Al2(HSal)(OH)4(H2O)4+with logkex-dAl-OH2correlation are3.0and5.2, respectively, indicating that the increase in the number of OH will promote the process of water-exchange reactions. This work promotes our further understanding of the dissolution and formation mechanism of aluminum minerals.