Trends in Aqueous Hydration Across the 4f Period Assessed by Reliable Computational Methods

Abstract

The geometric and electronic structures, as well as the thermodynamic properties of trivalent lanthanide hydrates {Ln(H(2)O)(8,9)(3+) and Ln(H(2)O)(8,9)(H(2)O)(12,14)(3+), Ln = La-Lu} have been examined using unrestricted density functional theory (UDFT), unrestricted Möller-Plesset perturbation theory (UMP2), and multiconfigurational self-consistent field methods (MCSCF). While Ln-hydrates with 2-5 unpaired f-electrons have some multiconfigurational character, the correlation energy lies within 5-7 kcal/mol across the period and for varying coordination numbers. As such DFT yields structural parameters and thermodynamic data quite close to experimental values. Both UDFT and UMP2 predict free energies of water addition to the Ln(H(2)O)(8)(3+) species to become less favorable across the period; however, it is a non-linear function of the surface charge density of the ion. UDFT further predicts that the symmetry of the metal-water bond lengths is sensitive to the specific f-electron configuration, presumably because of repulsive interactions between filled f-orbitals and water lone-pairs. Within the Ln(H(2)O)(8,9)(H(2)O)(12,14)(3+) clusters, interactions between solvation shells overrides this orbital effect, increasing the accuracy of the geometric parameters and calculated vibrational frequencies. Calculated atomic charges indicate that the water ligands each donate 0.1 to 0.2 electrons to the Ln(III) metals, with increasing electron donation across the period. Significant polarization and charge transfer between solvation shells is also observed. The relationship between empirical effective charges and calculated atomic charges is discussed with suggestions for reconciling the trends across the period.