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

Abstract

An intriguing absorption peak around ∼270 nm (4.59 eV) 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 480–490 nm (2.58–2.53 eV). 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 π-stacked ground-state water molecules, preferably non-hydrogen bonded, at relatively short intermolecular distances (around 2 Å). The lowest singlet excited state is characterised by an equilibrium distance of around 2 Å with a vertical absorption transition predicted at 4.5 eV. The excited π-stacked dimer has a large binding energy (∼1 eV). Therefore, near-ultraviolet light may favour the formation of structured water. Two relaxed side-hydrated π-stacked water molecules (a relaxed tetramer) constitute the smallest unique excimer-type fluorescent moiety consistent with the available experimental data.

Keywords:

• structured water
• π-stacked water
• excited states
• absorption spectra
• fluorescence spectra
• theoretical spectroscopy
• CASSCF/CASPT2

Acknowledgements

The research reported has been supported by the MEC-FEDER projects CTQ2010-14892 and CSD2007-0010 Consolider-Ingenio in Molecular Nanoscience. In honour of Trygve Helgaker's 60th birthday, his excellent lectures in the Sostrup Summer Schools are warmly remembered.

Notes

^aRef. [1].

^bRef. [5].

^cIt corresponds to the CASPT2 optimised intermolecular distance (R_min ) on S ₁ keeping fixed the geometry of the monomers.