Towards the understanding at the molecular level of the structured-water absorption and fluorescence spectra: a fingerprint of -stacked water

An intriguing absorption peak around approximate to 270nm (4.59eV) has been recurrently recorded in aqueous solutions of salts, sugars, amino acids, in the free-solute zone (exclusion zone) adjacent to various hydrophilic surfaces, as well as a transient in the conversion process of ice to water. The corresponding associated fluorescence has been observed in the interval 480-490nm (2.58-2.53eV). The spectroscopic features have been related to the presence of structured water but its nature remains incompletely understood. On the basis of high-level ab initio computations, the main absorption feature of structured water is assigned to the presence of two -stacked ground-state water molecules, preferably non-hydrogen bonded, at relatively short intermolecular distances (around 2 angstrom). The lowest singlet excited state is characterised by an equilibrium distance of around 2 angstrom with a vertical absorption transition predicted at 4.5eV. The excited -stacked dimer has a large binding energy (approximate to 1eV). Therefore, near-ultraviolet light may favour the formation of structured water. Two relaxed side-hydrated -stacked water molecules (a relaxed tetramer) constitute the smallest unique excimer-type fluorescent moiety consistent with the available experimental data.