Coordination and solvation of copper ion: Infrared photodissociation spectroscopy of Cu⁺(NH₃)_n (n = 3-8)

Coordination and solvation structures of the Cu⁺(NH ₃)_n ions with n = 3-8 are studied by infrared photodissociation spectroscopy in the NH-stretch region with the aid of density functional theory calculations. Hydrogen bonding between NH₃ molecules is absent for n = 3, indicating that all NH₃ molecules are bonded directly to Cu⁺ in a tri-coordinated form. The first sign of hydrogen bonding is detected at n = 4 through frequency reduction and intensity enhancement of the infrared transitions, implying that at least one NH ₃ molecule is placed in the second solvation shell. The spectra of n = 4 and 5 suggest the coexistence of multiple isomers, which have different coordination numbers (2, 3, and 4) or different types of hydrogen-bonding configurations. With increasing n, however, the di-coordinated isomer is of growing importance until becoming predominant at n = 8. These results signify a strong tendency of Cu⁺ to adopt the twofold linear coordination, as in the case of Cu⁺(H₂O)_n.