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 centre 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 centre 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 centre dot Ar but detected in Cu+(H2O)3 centre 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 incoordination is attributable to the inefficient 5s-Ad 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 centre dot Ar.Interestingly,the Ag+(H2O)4 centre dot Ar ions adopt not only the tricoordinated form but also the dicoordinated forms,which are absent in Ag+(H2O)3 centre 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.