Cluster spectroscopy, aided by ab initio theory, was used to determine the detailed structure of a complete hydration shell around an anion. Infrared spectra of size-selected O(2)-. (H(2)O)(n) (n = 1 to 4) cluster ions were obtained by photoevaporation of an argon nanomatrix. Four water molecules are required to complete the coordination shell. The simple spectrum of the tetrahydrate reveals a structure in which each water molecule is engaged in a single hydrogen bond to one of the four lobes of the pi* orbital of the superoxide, whereas the water molecules bind together in pairs. This illustrates how water networks deform upon accommodating a solute ion to create a distinct supramolecular species.

No abstract

To explore how a structured excess charge distribution affects the hydration of an anion, we report mid-IR, argon predissociation spectra for the hydrated superoxide cluster anions, O2−⋅(H2O)n, 1⩽n⩽5. This size range was chosen to establish the evolution of the structures through the putative shell closing [Weber et al., Science 287, 2461 (2000)] for superoxide hydration at the tetrahydrate. Whereas the observed bonding motifs for n⩽4 are those of single water molecules and dimeric subclusters bound to the ion, the pentahydrate spectrum displays strong bands in the region typically associated with ring modes of the water trimer. The present results reinforce the conclusion that the tetrahydrate adopts an especially robust structure in which each water molecule forms a single ionic H bond to one of the lobes of the π* highest occupied molecular orbital in superoxide.