Structures and Energetics of Hydrated Oxygen Anion Clusters

Hydration of the atomic oxygen radical anion is studied with computational electronic structure methods,
considering (O-)(H2O)n clusters and related proton-transferred (OH-)(OH)(H2O)n-1 clusters having n = 1−5.
A total of 67 distinct local-minimum structures having various interesting hydrogen bonding motifs are obtained
and analyzed. On the basis of the most stable form of each type, (O-)(H2O)n clusters are energetically favored,
although for n ≥ 3, there is considerable overlap in energy between other members of the (O-)(H2O)n family
and various members of the (OH-)(OH)(H2O)n-1 family. In the lower-energy (O-)(H2O)n clusters, the hydrogen
bonding arrangement about the oxygen anion center tends to be planar, leaving the oxygen anion p-like
orbital containing the unpaired electron uninvolved in hydrogen bonding with any water molecule. In
(OH-)(OH)(H2O)n-1 clusters, on the other hand, nonplanar arrangements are the rule about the anionic oxygen
center that accepts hydrogen bonds. No instances are found of OH- acting as a hydrogen bond donor. Those
OH bonds that form hydrogen bonds to an anionic O- or OH- center are significantly stretched from their
equilibrium value in isolated water or hydroxyl. A quantitative inverse correlation is established for all hydrogen
bonds between the amount of the OH bond stretch and the distance to the other oxygen involved in the
hydrogen bond.