Molecular dynamics investigation of alkali metal ions in liquid and aqueous ammonia

A polarizable potential model for M⁺-NH₃ interactions (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) is optimized based on the ab initio properties of the ion-ammonia dimers calculated at the MP2 level of theory. The optimized model reproduces the ab initio binding energies of M⁺(NH₃) _n (n = 2-4) and M⁺(NH₃)_n(H ₂O)_m (n, m = 1-3 and n + m ≤ 4) clusters and gives relative solvation free energies in liquid ammonia in good agreement with experimental data, without further adjustments. It also reproduces binding cooperativity in ion-ammonia and ion-ammonia-water clusters. The model is used in molecular dynamics simulations of isolated ions in liquid ammonia and in aqueous ammonia solutions with various ammonia molar fractions (0.0 ≤ x _NH3 ≤ 1.0). Simulations in liquid ammonia show coordination numbers of 4.0 for Li⁺, 5.3 for Na⁺, 6.1 for K ⁺, 6.7 for Rb⁺, and 7.7 for Cs⁺, in very good agreement with available experimental results. Simulations of ions in aqueous ammonia show preferential solvation by water in their first solvation shells and preferential solvation by ammonia in their second shells. Potentials of mean force are calculated between each ion and NH₃ in liquid water, and between each ion and H₂O in liquid ammonia. The results suggest that, in liquid water, Li⁺ and Na⁺ bind NH₃ in their second solvation shells only, while Cs⁺ binds NH₃ in its first solvation shell only (K⁺ and Rb⁺ ions show only weak affinity for NH₃ in water). In liquid ammonia, the ions bind H ₂O in their first solvation shells with an affinity following the trend Li⁺ > Na⁺ > K⁺ ≈ Rb⁺ > Cs⁺.