Infrared spectroscopy of Cu + ( H 2 O ) n and Ag + ( H 2 O ) n :Coordination and solvation of noble-metal ions

M + ( H 2 O ) n and M + ( H 2 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 + ( H 2 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 + ( H 2 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 H 2 O molecules is absent in Ag + ( H 2 O ) 3 ∙ Ar but detected in Cu + ( H 2 O ) 3 ∙ Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H 2 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 5 s - 4 d hybridization in Ag + , in contrast to the extensive 4 s - 3 d 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 H 2 O occupies the second shells of the tricoordinated Ag + and the dicoordinated Cu + , as extensive hydrogen bonding is observed in M + ( H 2 O ) 4 ∙ Ar . Interestingly, the Ag + ( H 2 O ) 4 ∙ Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag + ( H 2 O ) 3 ∙ Ar but revived at n = 4 . Size dependent variations in the spectra of Cu + ( H 2 O ) n for n = 5 - 7 provide evidence for the completion of the second shell at n = 6 , where the dicoordinated Cu + ( H 2 O ) 2 subunit is surrounded by four H 2 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.