Hydration Structure of ₁₀₂No²⁺: A Density Functional Theory-Molecular Dynamics Study

The hydration structure of No²⁺, the divalent cation of nobelium in water, was investigated by ab initio molecular dynamics (MD) simulations. First, a series of benchmark calculations were performed to validate the density functional theory (DFT) calculation methods for a molecule containing a No atom. The DFT-MD simulation of the hydration structure of No²⁺ was conducted after the MD method was validated by simulating the hydration structures of Ca²⁺ and Sr²⁺, whose behavior was previously reported to be similar to that of No²⁺. The model cluster containing M²⁺ (M = Ca, Sr, or No) and 32 water molecules was used for DFT-MD simulation. The results showed that the hydration distance of No²⁺ was intermediate between those of Ca²⁺ and Sr²⁺. This trend in the hydration distance is in good agreement with the elution position trend obtained in a previous radiochemical experiment. The calculated No-O bond lengths in the optimized structure of [No(H₂O)₈]²⁺ was 2.59 Å, while the average No-O bond length of [No(H₂O)₈]²⁺ in water by DFT-MD was 2.55 Å. This difference implies the importance of dynamic solvent effects, considering the second (and further) coordination sphere in the theoretical calculation of solution chemistry for superheavy elements.