Effect of Cations on Absorption Bands of First Electronic Transition of Liquid Water

Abstract

The effect of cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) on the first electronic transition (à ← X̃) of liquid water was investigated by attenuated total reflection far ultraviolet spectroscopy. To negate the effect of anions, aqueous solutions of 1 M alkali metal nitrates and bromides were compared at a temperature of 25 °C. It is found that the peak energy of the à ← X̃ band of water, which shows a marked red shift with decreasing hydrogen-bond strength, decreases with increasing cation size. The peak energies of the à ← X̃ band can be approximated by a linear function of the inverse of the ionic radii of the alkali metal cations, which indicates (according to the Born equation) that the first electronic transition of water is characterized by the solvation energy of the cations.