文摘
A polarizable potential model for M+鈥揘H3 interactions (M+ = Li+, Na+, K+, Rb+, Cs+) is optimized based on the ab initio properties of the ion鈥揳mmonia dimers calculated at the MP2 level of theory. The optimized model reproduces the ab initio binding energies of M+(NH3)n (n = 2鈥?) and M+(NH3)n(H2O)m (n, m = 1鈥? and n + m 鈮?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鈥揳mmonia and ion鈥揳mmonia鈥搘ater 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 鈮?xNH3 鈮?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 H2O in their first solvation shells with an affinity following the trend Li+ > Na+ > K+ 鈮?Rb+ > Cs+.