Structures of Th⁴⁺ aqueous solutions: insights from AIMD and metadynamics simulations

Solution chemistry of actinide ions is critical to understanding the solvation behaviors and hydrolysis process. Using tetravalent thorium ion Th⁴⁺ as a representative example, we investigate the local structures and dynamic behaviors of hydrated Th⁴⁺ ions by ab initio molecular dynamics (AIMD) simulations using the recently developed norm-conserving pseudopotentials and basis sets optimized for actinides (J.-B. Lu et al., J. Chem. Theory Comput. 2021, 17, 3360-3371). AIMD simulations reveal two distinct solvation shells, with the first shell comprising 9 water molecules at approximately r_Th-O = 2.50 Å and exhibiting a tricapped trigonal prism geometry. These conclusions are confirmed through metadynamics simulations and further structural analysis. AIMD simulations also show the slight effect of temperature and counterions on the structure of the solution. The structured solvation shells of the highly charged Th⁴⁺ ion with the specific geometry, distinct from the structure of liquid water, lead to corresponding structural changes in the hydrogen bond network in water. Additionally, beyond the solvent-shared ion pair (SIP) state observed in the unbiased AIMD simulations, the metadynamics simulations reconstruct a two-dimensional free energy surface that clearly indicates the potential stability of the contact ion pair (CIP) state in the system with Cl^- as a counterion. The findings in this work provide insights into the solution chemistry of actinides and serve as a reference for studying other actinide solution systems.