Water is the primary component in geological and biological systems, and the H-bonding environment in water plays a critical role in the processes mediated by water. Using the infrared spectroscopy, and neutron and x-ray diffraction measurements researchers have probed the nature of O-H bonds in water, and its structure around the dissolved molecules in water. However, the structure of water and its interactions with soluble molecules is still not well understood. Improved understanding on the chemical state of water in different molecular environments in aqueous solutions and at the interfaces is useful to predict the biogeochemical reactions and their rates in nature.
Using the soft x-ray endstation (soft x-ray endstation for environmental research, SXEER) built by our research group we conducted the x-ray absorption spectroscopy (specifically the near edge x-ray absorption fine structure, NEXAFS) of liquid water under ambient conditions. Our studies have indicated that the O NEXAFS spectrum of liquid water is significantly different from those of gas and solid forms, and further suggests that the spectra are sensitive to the variations in the H-bonding environment in water. The NEXAFS spectrum of gas phase water exhibits well-separated peaks corresponding to the O 1s excitations into the antibonding O-H 4a1 and 2b1 molecular orbitals at low energies, and transitions into the Rydberg orbitals at high energies. As water converts from the gas phase into one of the condensed phases, the spectra broaden with a distinct pre-edge feature in the latter. However, the pre-edge feature in the case of liquid water is very intense when compared to that of ice (Ih). Although the gas-phase and ice NEXAFS spectra were examined earlier, this is the first study where the NEXAFS spectra of liquid water and a detailed NEXAFS spectral analysis of different forms of water are presented. The spectrum of ice (for single crystal ice grown on Pt(111) surface) is collected by H. Ogasawara and A. Nilsson at the MAX Lab, Sweden.
The Density Functional Theory calculations of the O NEXAFS spectra together with the MD simulations on water clusters have reproduced the experimental NEXAFS spectra very closely, and also indicated that the electronic transitions at the O absorption edge are very sensitive to the H-bonding environment in water. Theoretical calculations on gas phase water and ice are reproduced using their previously known structures. When compared to the gas-phase water molecules, the spectrum of ice shows a strong shift of the spectral distribution into the continuum to form the conduction band. In addition the transitions to the antibonding 4a1 orbitals is weaker because of significant s character on the oxygen, which arises from the quasi-tetrahedral symmetry of water molecules in ice. The 1s → 2b1 transitions show appreciable intensity but are pushed up into the continuum and are seen as the enhancement of the intensity just above the ionization edge.
In the case of liquid water, we find that molecules with only certain types of coordination contribute to the intense pre-edge feature observed in liquid water. Based on the molecular dynamics simulations, we identified three different H-bond structures in liquid water with some variations in bond lengths and angles: 4HB structure, D-ASYM and A-ASYM. These structures represent water in 4 coordination with surrounding water molecules, water with a broken donating H-bond, and water with a broken accepting H-bond, respectively. The theoretical studies indicated that the D-ASYM water molecule exhibits the intense pre-edge feature in the NEXAFS spectra. The broken H-bonded configurations and resulting asymmetry in the tetrahedral nature of water increases the O-2p character in the pre-edge region.
Our current investigations are focused on understanding the influence of pH, ions and interfaces on the H-bonding networks and electronic structure of water in liquid water.