A theoretical investigation of the microsolvation of multivalent ions in clusters.

摘要

Trivalent lanthanide metals (Ln3+) are among the most spectroscopically active ions in the periodic table and are characterized by their exceptional ability to absorb and emit light in the ultraviolet, visible and near infra-red regions of the electromagnetic spectrum. These ions are sensitive to the nature of their ligands, as has been evidenced from their spectral properties in different environments. In an effort to predict the behaviour of lanthanide ions in different environments, solvated Ln 3+ ions in clusters, Ln3+(solvent)n, were investigated as a model system. Cluster studies provide an ideal means of monitoring progressive changes in the properties of the lanthanide ions with cluster size increases. The electronic, energetic and thermodynamic properties of Ln3+(H2O)n and Ln3+(CH 3CN)n clusters were simulated using a combination of quantum chemistry calculations, model potential development and Monte Carlo simulations, paying close attention to possible cluster-to-bulk transitions.;The Ln3+(H2O)n cluster binding enthalpies were found to be quite large, even at small cluster size, implying that these species should be stable under experimental conditions. However, small clusters have rarely been observed experimentally when they contain protic solvents and charge-reduced clusters, where the metal loses its 3+ charge, are observed instead. Thus, Eu3+(H2O)n cluster deprotonation was investigated as a possible explanation for the lack of experimental observation of small Ln3+(H2O)n clusters. The small clusters were found to favour loss of (solvated) hydronium ions from the cluster, explaining the experimentally-observed, charge-reduced clusters. Only recently (June 2006) was the experimental observation of large Ln3+(H 2O)n clusters (n > 15) reported. This is consistent with our prediction that deprotonation becomes less favourable with cluster size.;Finally, investigation of Ln3+(CH3CN) n clusters, using a similar methodology, reveals that formation of these clusters is also energetically favourable and that convergence to bulk, structural and thermodynamic properties are obtained at smaller cluster sizes than those observed in water clusters. Given that the thermodynamic properties of Ln 3+(CH3CN)n and large Ln3+(H 2O)n clusters have yet to be determined, the results herein may serve as benchmarks for future experimentation.;The properties of small Ln3+(H2O)n clusters obtained from quantum chemistry calculations indicate, much akin to other multi-valent Mq+(H2O)n clusters, that the metal ion-water interactions are predominantly electrostatic. Mutual polarization of both the ion and the water molecules accounts for the large Ln3+(H2O)n cluster binding energies and the resulting structural properties of the clusters. The quantum chemistry results were the basis for designing and parameterising polarizable model potentials for use in Monte Carlo simulations. The simulations revealed that bulk-like properties of Ln3+(H2O)n clusters, namely first-shell coordination numbers and bulk thermodynamic properties, are obtained at very large cluster sizes (n ≥ 64), thus showing that cluster studies are a good model for studying bulk solvation.