Infrared spectroscopy of hydrated sulfate dianions

The first infrared spectra of a multiply-charged anion in the gas phase are presented. The spectra of SO$_{4}^{2-}\cdot $(H$_{2}$O)$_{n}$, with $n$=3 to 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 solvent 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.