Coordination structures of the silver ion: Infrared photodissociation spectroscopy of Ag⁺(NH₃)_n (n = 3-8)

Infrared (IR) spectra are measured for Ag⁺(NH₃) _n with n = 3-8 in the NH-stretch region using photodissociation spectroscopy. The spectra of n = 3 and 4 exhibit absorption features only near the frequencies of the isolated NH₃, indicating that every NH ₃ molecule is coordinated individually to Ag⁺. For n ≥ 5, the occupation of the second shell is evidenced by lower-frequency features characteristic of hydrogen bonding between NH₃ molecules. Density functional theory and MP2 calculations are carried out in support of the experiments. A detailed comparison of the experimental and theoretical IR spectra reveals the preference for a tetrahedral coordination in the n = 5 and 6 ions. Likewise, most of the features observed in the spectra of n = 7 and 8 can be assigned to isomers containing a tetrahedrally coordinated inner shell as the basic structural motif. These results signify that the ammonia-solvated Ag⁺ ion has a propensity toward a coordination number of four and the resulting tetrahedral Ag⁺(NH₃)₄ complex forms the central core of further solvation process.