Theoretical Study of [Na(H₂O)_n]^- (n = 1−4) Clusters:  Geometries, Vertical Detachment Energies, and IR Spectra

Geometries, vertical detachment energies (VDEs), and IR spectra of [Na(H₂O)_n]^- (n = 1−4) have been investigated by an ab initio MO method at the correlated level. Water molecules are bound to Na^- via Na−H as well as hydrogen-bond interactions. The calculated VDEs are in good agreement with the recent photoelectron spectroscopy, and all observed bands are assignable to the 3²S(Na)−3S(Na^-) and 3P(Na)−3¹S(Na^-) type transitions perturbed by hydration. They are shifted to higher energy with increasing n by keeping their separation almost unchanged, which reflects the hydration structure of Na^-. We also report the calculated IR spectra that are informative about the ionic Na−H bonds and the hydrogen-bond network among water molecules in the clusters.