Flexibility and Regularity of the Hydration Structures of Ions by an Example of Na⁺: Nonempirical Insight

The stationary nonempirical simulations of [Na(H₂O)]⁺ clusters with n in the range of 28-51 carried out at the density functional level with a hybrid B3LYP functional and the Born-Oppenheimer molecular dynamics modeling of the size-selected clusters reveal the interrelated structural and energetic peculiarities of the sodium ion hydration structures. Surface, bulk, and transient structures are distinguished by different locations of the sodium nucleus (close to either the spatial center of the structure or one of its side faces) and a consistently changing coordination number (which typically equals five or six). The differences and correlations between the stationary and averaged dynamically changing configurations are quantified. The ⟨r_NaO⟩, mean Na-O distances for the first-shell water molecules, are found to depend both on the spatial character of the structure and the local coordination of sodium. The ⟨r_NaO⟩ values are compared to different experimental estimates, and the virtual discrepancy of the latter is explained based on the results of cluster simulations. Different coordination neighborhoods of the sodium ion are predicted depending on its local fraction in actual specimens.