Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): Coordination and solvation of noble-metal ions

摘要

M+(H2O)(n) and M+(H2O)(n)center dot 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+(H2O)(1)center dot 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+(H2O)(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 H2O molecules is absent in Ag+(H2O)(3)center dot Ar but detected in Cu+(H2O)(3)center dot Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H2O 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 H2O occupies the second shells of the tricoordinated Ag+ and the dicoordinated Cu+, as extensive hydrogen bonding is observed in M+(H2O)(4)center dot Ar. Interestingly, the Ag+(H2O)(4)center dot Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag+(H2O)(3)center dot Ar but revived at n=4. Size dependent variations in the spectra of Cu+(H2O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu+(H2O)(2) subunit is surrounded by four H2O molecules. The gas-phase coordination number of Cu+ is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes. (C) 2007 American Institute of Physics.