Electronic instability of isolated SO42− and its solvation stabilization

Despite its ubiquity in nature, SO42− as an isolated dianion has never been detected because of its electronic instability as a result of the two negative charges. This study shows how the first few waters solvate and stabilize an isolated SO42−, molecule-by-molecule, using photodetachment spectroscopy and theoretical calculations. We find that the minimum number of water required to stabilize a free SO42− is three. The first four waters bind tightly to SO42−, each forming two H-bonds with SO42− without inter-water H-bonding. The charges of the dianion are stabilized sufficiently that additional waters form only single H-bonds with SO42− and that inter-water H-bonding is observed starting at n=5.