Spectroscopic probing of local hydrogen bonding structures in liquid water

Abstract

Abstract We have studied the electronic structure of liquid water using x-ray absorption spectroscopy at the oxygen K edge. Since the x-ray absorption process takes less than a femtosecond, it allows probing of the molecular orbital structure of frozen, local geometries of water molecules at a timescale that has not previously been accessible. Our results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms, resulting in a pronounced pre-edge feature below the main absorption edge in the spectrum. Theoretical calculations of these spectra suggest that this feature originates from specific configurations of water, for which the H-bond is broken on the H-donating site of the water molecule. This study provides a fingerprint for identifying broken donating H-bonds in the liquid and shows that an unsaturated H-bonding environment exists for a dominating fraction of the water molecules. (Some figures in this article are in colour only in the electronic version) The hydrogen bond (H-bond) in liquid water holds the key to its peculiar behaviour, with implications for chemical, biological, and geological processes. In liquid water, the dynamical motion of the atoms at the picosecond timescale causes the H-bonds to break and reform, resulting in a statistical distribution of different coordinations for the water molecules. Water molecules in liquid and solid phases exhibit two types of O-H interaction: strong covalent O-H-bonds within the water molecule, and relatively weak H-bonds between the molecules. 9 Permanent address: RIKEN (The Institut