Spectral signatures of excess proton and electron accommodation motifs in small ionic water clusters

Obtaining clear molecular-level descriptions of the hydrated proton and hydrated electron represent two of the fundamental issues in modern-day physical chemistry. Bulk-phase spectral signatures of these species yield broad, continuous absorptions throughout the water bending and stretching regions, thereby making it difficult to identify precisely the accommodation motifs surrounding the excess charge defect. To circumvent this problem, small-water cluster analogs to the larger bulk-phase phenomena have been investigated. In the case of the hydrated proton, distinct spectral features associated with two different proton incorporation classes, the H3O+ "Eigen" and H3O2+ "Zundel" ion cores, are observed. The evolution of these motifs is monitored as a function of cluster size and it is observed that subtle changes to the hydration environment can result in extreme spectral shifts of the proton signature. In the case of the hydrated electron, cluster data reveal that the excess electron is bound to the surface of the neutral water core via a single water molecule. The infrared spectral signature of this unique water is distinct and reveals that it is attached to the water network through a double H-bond acceptor motif. It is observed that this spectral feature is quite robust and persists into larger water clusters containing upwards of 50 molecules!...