Evidence for Water Rings in the Hexahydrated Sulfate Dianion from IR Spectroscopy

Whereas isolated SO 4 2- is unstable, hydrated clusters of this dianion have been formed and investigated using a variety of different methods. Several structures of [SO 4 (H 2 O) 6 ] 2- have been proposed that account for its high stability in the gas phase. Zhou et al. [ J. Chem. Phys. 2006, 125, 111102] recently reported infrared spectra of [SO 4 (H 2 O) n ] 2- in the 540−1850 cm -1 region and assigned the spectrum of the hexahydrated ion to a T d symmetry structure in which all six water molecules donate two hydrogen bonds to the sulfate core. Here, an infrared spectrum of this ion in the hydrogen stretch region (2620−3840 cm -1 ) and B3LYP/AUG-cc-pVDZ calculations indicate that a significant population of these ions correspond to lower symmetry structures containing water rings in which each water molecule donates hydrogen bonds to both the sulfate dianion and a neighboring water molecule. These calculations indicate that inter-water hydrogen bonds are slightly favored over additional solvation of the dianion core. These results demonstrate that the low- and high-frequency infrared spectra of this ion provide complementary information, and the combination can greatly enhance structural elucidation of these hydrated ions.