The Hydrated Electron as a Pseudo-Atom in Cavity-Bound Water Clusters

Anionic water clusters, (H2O)n(-), of various sizes, n = 1-8, have been investigated using high-level ab initio calculations and the quantum theory of atoms in molecules, which provides a topological analysis of the electron density. The results of the current study indicate that the distribution of the excess electron is dependent on the geometry of the cluster. Non-nuclear attractors (NNAs), with associated pseudo-atomic basins and populations, are observed only in the highly symmetric clusters in which several non-hydrogen-bonded (NHB) hydrogen atoms are oriented toward a central cavity. For the latter cases, the non-nuclear attractor can be considered a pseudo-atom, possessing a significant portion of the excess electron within the cavity, consistent with the cavity-bound model of the solvated electron. In some cases, the population of the NNA is more than 0.2 electrons, and it contributes in excess of 20 kJ/mol to the energy of the system. Furthermore, the less symmetric systems, which tend to orient the NHB hydrogen atoms away from the center of the cluster, tend to delocalize the excess electron to a greater extent over several atoms at the surface of the cluster, consistent with the surface-bound model of the excess electron.