Infrared spectroscopy of cation-water complexes
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Cation-water complexes are produced in a pulsed supersonic expansion source. Metal containing ions are produced by laser vaporization and the electric discharge technique is used for protonated complexes. Mass-selected ions are investigated with infrared laser photodissociation spectroscopy and the method of rare gas predissociation. The infrared spectra of singly charged metal cation-water complexes show red shifts in the O-H stretching frequencies compared to corresponding stretches of the isolated water molecule. The red shift is caused by polarization of water induced by the metal cation. The symmetric stretch gains more intensity than that of the asymmetric stretch in the metal cation-water systems. These effects are more prominent for the doubly charged ions. Partially resolved rotational structures for the Sc+(H2O)Ar and Cr+(H2O) complexes show that the H-O-H bond angle is larger than it is in the free water molecule. Multiple argons on Mn+(H2O) and multiple waters on Zn+(H2O) produce various low energy isomers. Zn+(H2O)Ar shows the largest red shift in the O-H stretching frequencies, whereas for Cr+(H2O)Ar this shift is smaller in magnitude. For doubly charged metal-water complexes, the O-H stretches are observed roughly at the same positions. Fragmentation and the spectral pattern shows that the coordination of M2+(M = Sc, V, Cr) is filled with six ligands. Mixed protonated complexes of water and nitrogen have H3O+(N2)n structures with a partial proton sharing interaction. The proton affinity of benzene is higher than that of water, but in the [H(C6H6)(H2O)]+ complex the proton resides closer to water as an effect of a favorable solvation energy. The shared proton stretch for this complex shows a larger red shift than the O-H stretches of H3O+. The larger [H(C6H6)m(H2O)n]+ sizes have structures of protonated water clusters solvated by benzene. The shared proton stretch shows a larger blue shift in [H(C6H6)(H2O)2]+ compared to the corresponding stretch of H5O2+-Ar due to a greater polarization effect of benzene. The preferential site of protonation is always on water in the systems with multiple benzenes. The strength of the π-hydrogen bonds decreases as the system is progressively solvated by benzene. The coordination of H5O2+ is completed with four benzenes.