Infrared spectroscopy of hydrated sulfate dianions

We report the first infrared spectra of multiply-charged anions in the gas phase. The spectra of SO 4 2 − ∙ ( H 2 O ) 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 SO 4 2 − 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 .