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Abstract

We report the first infrared spectra of multiply-charged anions in the gas phase. The spectra of SO₄²⁻·(H2O)_n, with n=3–24, show four main bands assigned to two vibrations of the dianionic core, the water bending mode, and solvent libration. The triply degenerate SO₄²⁻ antisymmetric stretch vibration probes the local solvent symmetry, while the solvent librational band is sensitive to the hydrogen bonding network. The spectra and accompanying electronic structure calculations indicate a highly symmetric structure for the n=6 cluster and closure of the first solvation shell at n=12