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) x 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) x Ar but detected in Cu(+)(H(2)O)(3) x 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 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 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) x Ar. Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x 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.

Coordination and solvation structures of the Cu + (H 2 O) n ions with n = 1-4 are studied by infrared photodissociation spectroscopy and density functional theory calculations. Hydrogen bonding between H 2 O molecules is detected in Cu + (H 2 O) 3 and Cu + (H 2 O) 4 through a characteristic change in the position and intensity of OH-stretching transitions. The third and fourth waters prefer hydrogen-bonding sites in the second solvation shell rather than direct coordination to Cu +. The infrared spectroscopy verifies that the gas-phase coordination number of Cu + in Cu + (H 2 O) n is two and the resulting linearly coordinated structure acts as the core of further solvation processes.

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 NH(3) molecules is absent for n = 3, indicating that all NH(3) 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(+)(H(2)O)(n).

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The [Al(NH 3) n ] + ions with n = 1-5 are studied by infrared photodissociation spectroscopy and density functional theory calculations. 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-NH 2 ] +. Actually, the infrared spectra demonstrate that the [(H-Al-NH 2)(NH 3) n-1 ] + ions are predominant over [Al-(NH 3) n ] + for n ≥ 4, while the adduct structures dominate the spectra of [Al(NH 3) n •Ar] + for n = 1-3.