Elucidating Solvation Structures of Metal Ions by Infrared Spectroscopy and Quantum Chemical Calculations of Gas-phase Clusters

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Solvation structures of metal ions are explored by infrared (IR) photodissociation spectroscopy with the aid of density functional theory calculations. Selected examples of the results are as follows :1. For Al / NH_3 system, the inserted [H-Al-NH_2]^+ structure is calculated to be higher in energy than the adduct [Al-NH_3]^+ structure. However, incremental solvation stabilizes the inserted structure more efficiently than the adduct structure, because of a larger effective charge on the Al atom in [H-Al-NH2]+. Actually, the IR spectra show that the [(H-Al-NH_2)(NH_3)_<n-1>]^+ ions are predominant over [Al-(NH_3)_n]^+ for n【greater than or equal】4.2. The third H_2O attaches directly to Ag+ in a tri-coordinated form in Ag^+(H_2O)_3,while it occupies a hydrogen-bonding site in the second shell of the di-coordinated Cu+ in Cu^+(H_2O)_3. The preference of the tri-coordination is attributable to the inefficient 5s-4d hybridization in Ag+, in contrast to the extensive 4s-3d hybridization in Cu+ which retains the di-coordination. The fourth H_2O occupies the second shells of the tri-coordinated Ag+ and the di-coordinated Cu^+. Size dependent variations in the IR spectra of Cu+(H20)n, for n = 5-7 suggest that the di-coordinated structure acts as the core of further solvation processes.3. In Cu^+(NH_3)_3, three NH_3 molecules are bonded directly to Cu+ in a tri-coordinated form. The IR spectra of Cu^+(NH_3)_n with n = 4 and 5 suggest the coexistence of multiple isomers, which have different coordination numbers (2, 3, and 4). 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^+(H_2O)_n.