Spectroscopic probing of local hydrogen-bonding structures in liquid water

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.

Half or full core hole in density functional theory X-ray absorption spectrum calculations of water?

“…For liquid water it was demonstrated by Wernet et al 1 that the investigated molecular dynamics simulations, including ab initio Car-Parrinello molecular dynamics (CPMD) 23 , all resulted in a preponderance of DD species with two donating H-bonds. XA spectra of DD molecules are characterized by the same features typical of the spectrum of bulk ice with weak pre-and mainedges and an intense and broad post-edge around 540 eV 1, 8,9,21 Fig. 1 (left) Experimental XA spectrum of gas phase water from ref.…”

“…XAS (elsewhere also NEXAFS or XANES) 5 and the corresponding non-resonant x-ray Raman spectroscopy (XRS) 6 are well-established experimental techniques that have recently been extended to measurements of the local electronic structure of liquid water 1, [7][8][9][10][11] . Both XAS and XRS show major differences in the spectral profiles between bulk ice and liquid water 1,8,9 ; there is a well-defined pre-edge structure that can be seen in the spectrum of the liquid and is substantially reduced in the bulk ice spectrum; the remaining intensity is mainly due to proton disorder and minor defects 12 . The liquid water spectrum, with strong pre-edge (535 eV) and main-edge (537 eV) features, instead closely resembles that of the ice surface where a dominant fraction of the water molecules in the first half-bilayer has one free O-H bond 1, 13 .…”

“…The liquid water spectrum, with strong pre-edge (535 eV) and main-edge (537 eV) features, instead closely resembles that of the ice surface where a dominant fraction of the water molecules in the first half-bilayer has one free O-H bond 1, 13 . The spectrum of bulk ice, in which each molecule is tetrahedrally coordinated, is instead characterized by weak main edge (537 eV) and strong post edge (540 eV) features 1,8 ; these experimental differences already indicate that the liquid cannot be dominated by fully coordinated molecules. In ref.…”

ERKALE—A flexible program package for X‐ray properties of atoms and molecules

“…As is well known, the acceptor spectrum bears a close resemblance to the monomer spectrum, whereas the donor spectrum is substantially affected by the formation of the hydrogen bond [141] ; the same feature is seen also in the case of alcohols. [142] The experimental XAS edge of water in the gas phase is roughly at 534 eV, [143] thus the XCH method (without corrections for functional dependence, etc.) underestimates the transition energy by some 5.5 eV.…”

Fingerprints in IR OH vibrational spectra of H2O clusters from different H-bond conformations by means of quantum-chemical computations

“…[1][2][3][4][5][6][7][8] The exploration of structural and binding properties of water clusters may provide a route to understand bulk water in its liquid or solid phase. [1,9] Thus many studies have been presented on the structure of water clusters from small to large sizes both experimentally and theoretically. [10][11][12][13][14][15][16][17][18][19][20] For small clusters with up to ten water molecules, most computational ab initio investigations predict approximately the same geometries, which are consistent with experimental results.…”