Coordination and solvation of noble metal ions: Infrared spectroscopy of Ag⁺(H₂O)_n

Vibrational spectra of mass-selected Ag⁺(H₂O) _n ions are measured by infrared photodissociation spectroscopy and analyzed with the aid of density functional theory calculations. Hydrogen bonding between H₂O molecules is found to be absent for cold Ag ⁺(H₂O)₃, but detected for Ag⁺(H ₂O)₄ through characteristic changes in the position and intensity of OH-stretching transitions. The third H₂O coordinates directly to Ag⁺, but the fourth H₂O prefers solvation through hydrogen bonding. The preference of the tri-coordinated form is attributed to the inefficient 5s-4d hybridization in Ag⁺, in contrast to the efficient 4s-3d hybridization in Cu⁺. For Ag ⁺(H₂O)₄, however, di-coordinated isomers are identified in addition to the tri-coordinated one.