Infrared spectroscopy of Cu+ (H2 O)n and Ag+ (H2 O)n: Coordination and solvation of noble-metal ions

M+ (H2 O)n and M+ (H2 O)n Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions. These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer. Density functional theory calculations are also carried out for analyzing the experimental IR spectra. Partially resolved rotational structure observed in the spectrum of Ag+ (H2 O)1 Ar indicates that the complex is quasilinear in an Ar- Ag+ -O configuration with the H atoms symmetrically displaced off axis. The spectra of the Ar-tagged M+ (H2 O)2 are consistent with twofold coordination with a linear O- M+ -O arrangement for these ions, which is stabilized by the s-d hybridization in M+. Hydrogen bonding between H2 O molecules is absent in Ag+ (H2 O)3 Ar but detected in Cu+ (H2 O)3 Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H2 O attaches directly to Ag+ in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu+. The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag+, in contrast to the extensive 4s-3d hybridization in Cu+ which retains the dicoordination. This is most likely because the s-d energy gap of Ag+ is much larger than that of Cu+. The fourth H2 O occupies the second shells of the tricoordinated Ag+ and the dicoordinated Cu+, as extensive hydrogen bonding is observed in M+ (H2 O)4 Ar. Interestingly, the Ag+ (H2 O)4 Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag+ (H2 O)3 Ar but revived at n=4. Size dependent variations in the spectra of Cu+ (H2 O)n for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu+ (H2 O)2 subunit is surrounded by four H2 O molecules. The gas-phase coordination number of Cu+ is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes.