Strengthening of the Coordination Shell by Counter Ions in Aqueous Th⁴⁺ Solutions

The presence of counterions in solutions containing highly charged metal cations can trigger processes such as ion-pair formation, hydrogen bond breakages and subsequent re-formation, and ligand exchanges. In this work, it is shown how halide (Cl^–, Br^–) and perchlorate (ClO₄^–) anions affect the strength of the primary solvent coordination shells around Th⁴⁺ using explicit-solvent and finite-temperature ab initio molecular dynamics modeling methods. The 9-fold solvent geometry was found to be the most stable hydration structure in each aqueous solution. Relative to the dilute aqueous solution, the presence of the counterions did not significantly alter the geometry of the primary hydration shell. However, the free energy analyses indicated that the 10-fold hydrated states were thermodynamically accessible in dilute and bromide aqueous solutions within 1 kcal/mol. Analysis of the results showed that the hydrogen bond lifetimes were longer and solvent exchange energy barriers were larger in solutions with counterions in comparison with the solution with no counterions. This implies that the presence of the counterions induces a strengthening of the Th⁴⁺ hydration shell

Importance of Counteranions on the Hydration Structure of the Curium Ion

Using density functional theory based ab initio molecular dynamics and metadynamics, we show that counterions can trigger noticeable changes in the hydration shell structure of the curium ion. On the basis of the free energies of curium–water coordination, the eight-fold coordination state is dominant by at least 98% in the absence of counteranions and in the presence of chloride and bromide counteranions. In addition, the solvent hydrogen bond (HB) lifetimes are relatively longer. In contrast, the solvent hydrogen bond (HB) lifetimes are relatively shorter in the presence of perchlorate counteranions, with the nine-fold and eight-fold states existing in an 8/2 ratio, which is in good agreement with the reported ratio measured by X-ray scattering experiments. To our knowledge, this is the first time that molecular simulations have shown that counteranions can directly affect the first hydration shell structure of a cation