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The structural and energetic features of a variety of gas-phase aluminum ion hydrates containing up to 18 water molecules have been studied computationally using density functional theory. Comparisons are made with experimental data from neutron diffraction studies of aluminum-containing crystal structures listed in the Cambridge Structural Database. Computational studies indicate that the hexahydrated structure Al[H(2)O](6)(3+) (with symmetry T(h)()), in which all six water molecules are located in the innermost coordination shell, is lower in energy than that of Al[H(2)O](5)(3+).[H(2)O], where only five water molecules are in the inner shell and one water molecule is in the second shell. The analogous complex with four water molecules in the inner shell and two in the outer shell undergoes spontaneous proton transfer during the optimization to give [Al[H(2)O](2)[OH](2)](+).[H(3)O(+)](2), which is lower in energy than Al[H(2)O](6)(3+); this finding of H(3)O(+) is consistent with the acidity of concentrated Al(3+) solutions. Since, however, Al[H(2)O](6)(3+) is detected in solutions of Al(3+), additional water molecules are presumed to stabilize the hexa-aquo Al(3+) cation. Three models of a trivalent aluminum ion complex surrounded by a total of 18 water molecules arranged in a first shell containing 6 water molecules and a second shell of 12 water molecules are discussed. We find that a model with S(6) symmetry for which the Al[H(2)O](6)(3+) unit remains essentially octahedral and participates in an integrated hydrogen bonded 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[H(2)O](6)(3+) do not appear to be sufficiently strong to orient the dipole moments of these second-shell water molecules toward the Al(3+) ion. 相似文献
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Hydrogen bonding and the electron-withdrawing or electron-donating characteristics of substituent groups that are neighboring to epoxide groups can affect the reactivity of the epoxide ring. The crystal structure ofcis-2,3-epoxycyclooctanol has been determined as a saturated eight-membered ring compound in which a hydroxyl group is attached to the C(1) atom that is adjacent to a 2,3-fused epoxy ring. The findings are that the longer epoxide C-O bond (and hence the one expected to be more readily broken) is the one farther from the hydroxyl group [1.462(1) å versus 1.447(1) å] and that the optimal hydrogen bonding is to an adjacent molecule radier than within the molecule. The shortest C-C bond is that of the epoxide group; the bond adjacent to it (on the side farther from the hydroxyl group) is the next shortest. 相似文献
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Charles W. Bock George D. Markham Amy K. Katz Jenny P. Glusker 《Theoretical chemistry accounts》2006,115(2-3):100-112
Structural features of clusters involving a metal ion (Li+, Na+, Be2+, Mg2+, Zn2+, Al3+, or Ti4+) surrounded by a total of 18 water molecules arranged in two or more shells have been studied using density functional theory.
Effects of the size and charge of each metal ion on the organization of the surrounding water molecules are compared to those
found for a Mg[H2O]62+• [H2O]12 cluster that has the lowest known energy on the Mg2+• [H2O]18 potential energy surface (Markham et al. in J Phys Chem B 106:5118–5134, 2002). The corresponding clusters with Zn2+ or Al3+ have similar structures. In contrast to this, clusters with a monovalent Li+ or Na+ ion, or with a very small Be2+ ion, differ in their hydrogen-bonding patterns and the coordination number can decrease to four. The tetravalent Ti4+ ionizes one inner-shell water molecule to a hydroxyl group leaving a Ti4+(H2O)5 (OH−) core, and an H3O+• • • H2O moiety dissociates from the second shell of water molecules. These observations highlight the influence of cation size and
charge on the local structure of hydrated ions, the high-charge cations causing chemical changes and the low-charge cations
being less efficient in maintaining the local order of water molecules.
Electronic Supplementary Material: Supplementary material is available for this article at http://dx.doi.org/10.1007/S00214-005-0056-2. 相似文献
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The crystal structure of hydrated manganese citrate, [Mn(H2O)6] [Mn C6H5O7(H2O)]22H2O has been determined. The crystals are monoclinic, space group P21/n cell dimensions a = 20.575 ± 0.005, b = 6.755 ± 0.002, c = 9.230 ± 0.002 Å, β = 96.74 ± 0.01°. The structure is isomorphous with that of the magnesium salt for which the structure has been determined4. There are two [Mn(H2O)6]2+ ions, four [Mn C6H5O7(H2O)] ions and four water molecules per unit cell. Each citrate ion forms a tridentate chelate to one manganese ion. A correlation of Mn2+ ? H distances for manganese citrate, found in this crystallographic study, was made with those determined by NMR studies in solutions in the absence and presence of the enzyme aconitase, and indicated that the assumption that citrate chelates to enzyme-bound manganese ion fits all the available data. 相似文献