The Arrangement of First- and Second-Shell Water Molecules in Trivalent Aluminum Complexes: Results from Density Functional Theory and Structural Crystallography

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• 作者：Charles W. Bock ; George D. Markham ; Amy K. Katz ; and Jenny P. Glusker
• 刊名：Inorganic Chemistry
• 出版年：2003
• 出版时间：March 10, 2003
• 年：2003
• 卷：42
• 期：5
• 页码：1538 - 1548
• 全文大小：177K
• 年卷期：v.42,no.5(March 10, 2003)
• ISSN：1520-510X

文摘

The structural and energetic features of a variety of gas-phase aluminum ion hydrates containing up to 18 watermolecules have been studied computationally using density functional theory. Comparisons are made with experimentaldata from neutron diffraction studies of aluminum-containing crystal structures listed in the Cambridge StructuralDatabase. Computational studies indicate that the hexahydrated structure Al[H₂O]₆³⁺ (with symmetry T_h), in whichall six water molecules are located in the innermost coordination shell, is lower in energy than that of Al[H₂O]₅³⁺·[H₂O], where only five water molecules are in the inner shell and one water molecule is in the second shell. Theanalogous complex with four water molecules in the inner shell and two in the outer shell undergoes spontaneousproton transfer during the optimization to give {Al[H₂O]₂[OH]₂}⁺·[H₃O⁺]₂, which is lower in energy than Al[H₂O]₆³⁺;this finding of H₃O⁺ is consistent with the acidity of concentrated Al³⁺ solutions. Since, however, Al[H₂O]₆³⁺ isdetected in solutions of Al³⁺, additional water molecules are presumed to stabilize the hexa-aquo Al³⁺ cation. Threemodels of a trivalent aluminum ion complex surrounded by a total of 18 water molecules arranged in a first shellcontaining 6 water molecules and a second shell of 12 water molecules are discussed. We find that a model withS₆ symmetry for which the Al[H₂O]₆³⁺ unit remains essentially octahedral and participates in an integrated hydrogenbonded network with the 12 outer-shell water molecules is lowest in energy. Interactions between the 12 second-shell water molecules and the trivalent aluminum ion in Al[H₂O]₆³⁺ do not appear to be sufficiently strong to orientthe dipole moments of these second-shell water molecules toward the Al³⁺ ion.