Preparation and photoelectron spectrum of the ‘missing’ cluster

We report a rational synthetic approach for the preparation of the negatively charged water tetramer anion, the only member of the homogeneous ( H 2 O ) n - series that has been curiously ‘missing’ in previous reports of these cluster anions. The synthesis was carried out using argon cluster-mediated anion formation. The identity of the ( H 2 O ) 4 - cluster is confirmed using negative ion photoelectron spectroscopy, and the resulting spectrum suggests the presence of three isomeric forms for this species.