| Aluminum is the most abundant metal in the earth’s crust. Due to the acidification of the surface waters and acidic precipitation, the dissolution of a large amount of aluminum mineral leads to the elevated levels of dissolved aluminum in aquatic ecosystems. This not only influences the transport, bioavailability and toxicity of aluminum, but also has a strong impact on the geochemical cycle of other elements as well as the transfer and fate of organic pollutants. The study on aluminum chemistry in aqueous solutions is closely connected with the dissolution, transfer and transformation of aluminum, which is helpful to understand the reactivity and toxicology of aluminum in the environment and organism. Because of the small size and high charge, Al3+is easy to form a series of hydroxyl polyaluminums by hydrolysis-polymerization process. Obviously, the process is directly influenced by the environmentally ubiquitous inorganic anions and organic ligands. Therefore, the study on the hydrolysis-polymerization mechanism as well as the influence of other ions becomes the key topics of aluminum chemistry in natural water. It should be noted that the three problems exist in this field at present:(1) As the simplest ligand substitution reaction, water-exchange reaction exerts control over the hydrolysis-polymerization process of aluminum species and the whole reaction on mineral surface. For the water exchange investigation, the water-exchange rate kex is the most important parameter. Although the keX of some aluminum species has been determined using variable-temperature 17O NMR, the experiment can not be applied for the most aluminum species due the limitation of experimental technique. The theoretical computation is another important approach for the study on water-exchange reaction, but the kex can not also be directly obtained by the theoretical technique. In this case, the establishment of reliable structure-effect relationship between theoretical computation and experiment becomes the key to study aluminum species; (2) It is difficult to confirm the mechanism for the ligand substitution reaction of aluminum species. The activation volume △V≠ observed by variable-pressure 17O NMR is a reliable criteria to differentiate mechanisms experimentally, but the experiment fails to implement in most cases. Of all the aluminum species, the △V≠ is determined experimentally only for Al(H2O)63+ and Keggin-GaAl127+ so far. The obtained △V≠ values show the activation mechanism with dissociate character. However, we can not ascertain whether all the other ligand substitution reactions of aluminum species follow the dissociative mechanism and whether the substitution mechanism changes as a function of ligand as well as the types of reactions; (3) The previous study on synthesis of polyaluminums suggests that the counter anions in aqueous solution have an effect on the formation of polyaluminums. How do the counter anions influence the formation of polyaluminums? The answers to these questions would greatly advance the study on environmental aluminum chemistry. In this dissertation, the influence of the diverse ligands on the structures and water exchange reactions of aqueous monoaluminum are discussed; the structures and the water exchange reactions of Core-links tridecamer and Flat aluminum tridecamer are explored; the influence of the counter anions on structure and pKa of monoaluminum are inspected. There are seven chapters as follows:1. IntroductionThe study of aqueous aluminum species in environment is of great environmental significance. The investigation on water-exchange reactions of aluminum species in aqueous solutions and the influence of various ions and ligands 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 and biological fluids. In this chapter, the significance of aqueous aluminum species in environment and nanomineralization has been described; the experimental and theoretical studies on the water-exchange reaction as well as the structure-effect relationship are reviewed; the advancement in the theoretical studies of aluminum chemistry is summarized.2. Density functional theory studies on the structures and water-exchange reactions of aqueous aluminum-fluoride complexesThe density functional theory calculation is carried out to investigate the structures, 19F and 27AlNMR chemical shifts of aqueous Al-F complexes and their water-exchange reactions. The following investigations are performed in this paper:(1) The microscopic properties of typical aqueous Al-F complexes are obtained at the level of B3LYP/6-311+G(d,p). AI-OH2 bond lengths increase with F" replacing inner-sphere H2O progressively, indicating labilizing effect of F- ligand. The Al-OH2 distance trans to fluoride is longer than other Al-OH2 distance, accounting for trans effect of F" ligand. 19F and 27Al NMR chemical shifts are calculated using GIAO method at the HF/6-311+G(d,p) level relative to F(H2O)6- and Al(H2O)63+references, respectively. The results are consistent with available experimental values; (2) The dissociative (D) activated mechanism is observed by modeling water-exchange reaction for [Al(H2O)6-iFi](3-i)+(i= 1-4). The activation energy barriers are found to decrease with increasing F- substitution, which is in line with experimental rate constants (kex). The log kex of AlF3(H2O)30 and AlF4(H2O)2- are predicted by three ways. The results indicate that the correlation between log kex and Al-0 bond length as well as the given transmission coefficient allows experimental rate constants to be predicted, whereas the correlation between log kex and activation free energy is poor; (3) The environmental significance of this work is elucidated by the extension toward three fields, that is, polyaluminum system, monomer Al-organic system and other metal ions system with high charge-to-radius ratio.3. Density functional theory studies on the structures and water-exchange reactions of aqueous aluminum-oxalate complexesThe density functional theory calculation is carried out to investigate the structures and water-exchange reactions of Al(III)-oxalate complexes. The present work included the following three studies:(1) The optimized geometries of Al(C2O4)(H2O)4+ and Al(C2O4)2(H2O)2- at the level of B3LYP/6-311+G(d,p) suggest that the bond length of Al-OH2 cis to C2O42- ligand is much longer than Al-OH2 trans to C2O42- in Al(C2O4)(H2O)4+. For Al(C2O4)2(H2O)2-, the energy of cis isomer is close to trans isomer with the consideration of sufficient solvent effects, implying that both the isomers coexists in aqueous solutions. The computational 27Al NMR and 13C NMR shifts using HF GIAO method and 6-311+G(d,p) basis set indicate that the 27Al NMR and 13C NMR shifts including both the explicit and bulk solvent effects are consistent with the experimental values available; (2) The water-exchange reactions of Al(III)-oxalate complexes are simulated at the same computational level. The results show that the water exchange reactions follow the dissociative mechanism and the solvent effect has a strong influence on the activation energy barriers. The barriers are sensitive to the location of the adding explicit solvent molecules and the forming double hydrogen bonds to a coordinated H2O and a carbonyl group yields the lowest barrier. According to the correlation between Al-OH2 bond length and log kex, water-exchange rates of Al(C2O4)2(H2O)2- are speculated. The computed values indicate that the bound water of trans isomer is more labile than cis isomer; (3) The significances of the present study in environmental chemistry are elucidated. The influence of various organic ligand in environment on aluminum chemistry can be explored by extending this study.4. Density functional theory studies on the structural characteristics and water-exchange reactions of core-links aluminum tridecamer (C-Al13)The structures and water-exchange reactions of core-links aluminum tridecamer (C-Al13) are investigated using density functional theory. (1) The geometries of core-links aluminum tridecamer were optimized with the consideration of both the explicit and bulk solvent effects at the level of B3LYP/6-31G(d). For fully protonated core-links aluminum tridecamer Al13(OH)3o(H20)189+(C-Al139+) the Al-OH2 bond lengths are longer than those of other polyaluminum species reported previously, such as Al6(OH)186+and cage-like Keggin-Al13 (K-Al137+), implying the higher coordinated water reactivity. The computed 27Al NMR chemical shifts based on the optimized structures at the HF/6-311+G(d, p) level of theory are in accord with experimental values available, suggesting the appropriateness of computational models; (2) The water-exchange reactions of structurally distinct coordinated water on C-Al139+were simulated, respectively. The results indicate that water exchange proceeds via dissociative mechanism and η-OH2(2) is the most reactive site. Moreover, the water-exchange rate constants (kex) at different sites for C-Al13 were predicted according to the given transmission coefficients and it is concluded that the deprotonation causes the acceleration of water exchange; (3) The geochemical significance of C-AI13 is elucidated based on the computational results, including the relevance to gibbsite surface and the proposal of the possible formation mechanism from C-Al13 to K-AI13.5. Density functional theory studies on the structure and water-exchange reactions of the aluminum polyoxocation Flat-Al13(F-Al13)The structures and water-exchange reactions for flat-Al13 (F-Al13) complexes in aqueous solution have been investigated using density functional study. The present work focuses on the following three aspects:(1) F-Al13 are optimized with the consideration of solvent effect at the level of B3LYP/6-31G(d) and the geometries obtained are in agreement with X-ray diffraction data previously observed for the corresponding inorganic salt crystal, suggesting the appropriateness of the computational approach. The computed 27Al NMR chemical shifts using Hartree Fock Giao method are comparable to the experimental values available; (2) The water-exchange reactions on the two distinct bound water sites (η-OH2(1) and η-OH2(2)) were simulated and the dissociative activation mechanism is elucidated. The results show that η-OH2(1) is more labile than η-OH2(2), which is consistent with the trans effect of hydroxyl. According to the simulation on a series of deprotonated species, we conclude that the deprotonation has little influence on the energy barrier for η-OH2(1), but causes the considerable increase in reactivity for η-OH2(2). With respect to the structural properties and water-exchange reaction, the comparison of F-Al13 with other aluminum species is performed. It is clear that the water-exchange rate of polyaluminum is much higher than that of monoaluminum and the order of lability of three aluminum tridecamers is F-Al13> C-Al13> K-Al13; (3) The geochemical significances of the present work are discussed. The structural analogy to mineral surface allows F-Al13 to act as a suitable model compound and the computational results are helpful to understand the formation mechanism of brucite.6. Density functional theory studies on the mechanism for the substitution of F" into Al(Ⅲ) complexes in aqueous solutionThe mechanisms for the substitution of an aqua ligand with F- in monomeric Al complexes were studied with density functional theory. Typical mechanisms are modeled to determine the preferred substitution pathway according to the activation energy barriers. The present computational results are in favor of interchange associative (Ia) mechanism for the substitution of F" into Al(H2O)63+, whereas interchange dissociative (Id) mechanism is preferred for the substitution into Al(H2O)5(OH)2+, which is in agreement with the previous experimental findings. This implies the mechanistic changeover from Ia to Id induced by the spectator hydroxyl ligand. Like the water-exchange reaction, the substitution rate is accelerated by OH" ligand. The difference of the computational and experimental activation enthalpy values is interpreted as the DFT errors in energy and the deviation of transmission coefficient from unity.7. Density functional theory studies on the interaction between monomeric aluminium and chloride ion and pKa calculations in Aqueous SolutionIn this chapter the interaction of monomeric aluminium and chloride ion in aqueous solution is investigated by density functional theory calculations. The computational results show that it is difficult for Cl- to enter the inner-coordination shell of aluminium complexes by replacing the bound water molecules, independent of pH and the concentration of Cl-. However, pH and the concentration of Cl- might influence the conformations, bond lengths and natural charge populations of monomeric aluminium complexes to a given extent. Based on the computed Gibbs energies, pKa values of various hydrolysis species in the presence and absence of Cl- are evaluated, respectively. It is concluded that pKa increases with the introduction of Cl-. |
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