The effect of metal cations on the nature of the first electronic transition of liquid water as studied by attenuated total reflection far-ultraviolet spectroscopy.

The first electronic transition (Ã←X̃) of liquid water was studied from the perspective of the hydration of cations by analyzing the attenuated total reflection far-ultraviolet (ATR-FUV) spectra of the Group I, II, and XIII metal nitrate electrolyte solutions. The Ã←X̃ transition energies of 1 M electrolyte solutions are higher (Li(+): 8.024 eV and Cs(+): 8.013 eV) than that of pure water (8.010 eV) and linearly correlate with the Gibbs energies of hydration of the cations. The increases in the Ã←X̃ transition energies are mostly attributable to the hydrogen bond formation energies of water molecules in the ground state induced by the presence of the cations. The deviation from the linear relation was observed for the high charge density cations, H(+), Li(+), and Be(2+), which reflects that the electronic energies in the excited states are also perturbed. Quantum chemical calculations show that the Ã←X̃ transition energies of the water-cation complexes depend on the hydration structures of the cations. The calculated Ã←X̃ transition energies of the water molecules hydrating high charge density cations spread more widely than those of the low charge density cations. The calculated transition energy spreads of the water-cation complexes directly correlate with the widths of the Ã←X̃ transition bands measured by ATR-FUV spectroscopy.