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In this study, the 27 Al NMR chemical shifts and relative stabilities of monomeric Al 3+ hydrolytic species with different coordination structures in aqueous solution are systematically investigated by using the density functional theory quantum chemical cluster model (DFT-CM) at the B3LYP/6-311+G(d,p) level. The main work includes: the static configurations of 20 possible existing monomeric Al 3+ hydrolytic species from Al 3+ to Al(OH) 4 − are optimized, and their 27 Al NMR shieldings are calculated; the dehydration reaction pathways for typical monomeric Al 3+ hydrolytic species are modeled, and the dominant forms of the intermediate hydrolytic species of Al(OH) 2+ , Al(OH) 2 + , and Al(OH) 3 0 are analyzed based on the Gibbs free energy changes of the dehydration reactions. The important role of the tetracoordinated Al(H 2 O)-(OH) 3 0 in the formation mechanism of the polynuclear Keggin-Al 13 is discussed. This work provides valuable references for further studying the formation and transformation mechanisms of the aqueous monomeric and polymeric Al species.

The kinetic mechanism of spontaneous aluminum ion (Al3+) hydrolysis reaction in aqueous solution is investigated using the density functional theory–quantum chemical cluster model method. Three typical reaction pathways for the spontaneous Al3+ hydrolysis reaction are modeled, including (1) the traditional spontaneous proton dissociation on the Al3+ inner-shell coordinated waters; (2) the conventional bulk water-assisted proton dissociation; and (3) the second-shell water-assisted synergistic dissociation of the protons on the Al3+ inner-shell waters. The results show that the electrostatic effects between Al3+ and its coordinated waters alone cannot fully account for the proton loss on an inner-shell coordinated water. It is suggested that the main reaction pathway for natural hydrolysis of aqueous Al3+ is the second-shell water-assisted synergistic proton dissociation, in which the participation of the second hydration shell is crucially important. The calculated synergistic proton dissociation rate constant, k H + = 1.14 × 105 s–1, is in close agreement with the experimental results (1.09 × 105 s–1 and 7.9 × 104 s–1). The first hydrolysis equilibrium constant pK a1 of Al3+ is calculated as 5.82, also consistent with the literature value of 5.00. This work elucidates the molecular mechanism of the spontaneous Al3+ hydrolysis reaction in natural waters and has important environmental implications.

Density functional theory (DFT) calculations combined with cluster models are performed at the B3LYP/6-311+G(d,p) level for investigating the solvent effects in Al(HO) water-exchange reactions. A "One-by-one" method is proposed to obtain the most representative number and arrangement of explicit HOs in the second hydration sphere. First, all the possible ways to locate one explicit HO in second sphere (N' = 1) based on the gas phase structure (N' = 0) are examined, and the optimal pathway (with the lowest energy barrier) for N' = 1 is determined. Next, more explicit HOs are added one by one until the inner-sphere is fully hydrogen bonded. Finally, the optimal pathways with N' = 0-7 are obtained. The structural and energetic parameters as well as the lifetimes of the transition states are compared with the results obtained with the "Independent-minimum" method and the "Independent-average" method, and all three methods show that the pathway with N' = 6 may be representative. Our results give a new idea for finding the representative pathway for water-exchange reactions in other hydrated metal ion systems.

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