Ammonia Solvation of Alkaline Earth Dications: Mg(II), Ca(II), Sr(II), and Ba(II); Hybrid Density Functional Theory Born–Oppenheimer Molecular Dynamics Studies

Structural and dynamical properties of Sr²⁺ and Ba²⁺ dications in ammonia microsolvation environments were studied through hybrid density functional theory Born-Oppenhemier molecular dynamics of [Sr(NH₃)_n]²⁺ and [Ba(NH₃)_n]²⁺ clusters with n = 2, 3, 4, 5, 6, 8, 10, and 27. The largest cluster models were used to explore bulk phase solvation of Sr²⁺ and Ba²⁺ in liquid ammonia for which experimental data are available. Results are discussed in the light of previous results obtained for the [Mg(NH₃)_n]²⁺ and [Ca(NH₃)_n]²⁺ systems with the same methodology. Vibrational and EXAFS spectra are reported for the first time for [Sr(NH₃)_n]²⁺ and [Ba(NH₃)_n]²⁺ systems. It was found that alkaline earth dications have coordination numbers (CN) in ammonia as follows: Mg²⁺ (6) < Ca²⁺ (8) < Sr²⁺ (8.3) < Ba²⁺ (9.4). The coordination structures found turn out to be rather flexible with CN greater than six and these structures depart from the simple geometry of hexamine in the solid phase.