Opposite effects of successive hydration shells on the aqua ion structure of metal cations
Résumé
The conformation adopted by water molecules around a metal ion in aqueous solution can be approached from a discrete representation of the solvent by studying the structure and stability of microsolvation clusters including a different number of water molecules. In this contribution we present a Quantum Mechanics and ab initio Molecular Dynamics study on how the arrangement of water molecules around Mg2+ and Al 3+ shows preferentially a Th or S6 symmetry depending on the number of hydration shells explicitly included in the calculation. The behavior observed for both cases is the following: a) For a cluster composed of the metal ion plus a first hydration shell the preferred geometry shows a Th symmetry. b) When a second hydration shell is added, a minimum can be found with a S6 disposition of water molecules around the metal center. c) However, if a third hydration shell is considered a Th arrangement of water molecules around the central ion is strongly favored. The structures of the different solvation complexes studied in this work are characterized. The preference for a Th or S6 distribution of water ligands is rationalized in terms of total energies, interaction energies and the hydrogen bond network.
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