High-resolution soft X-ray spectroscopy revives Röntgen's water structure model

The molecular structure of liquid water has been the subject of intense debate for decades. In 1892, German physicist W. C. Röntgen, who became famous for his discovery of X-rays, published a paper proposing a "mixture model" according to which liquid water consists of two kinds of molecules: a tetrahedral ice-like structure, and another more loosely arranged structure. In 1933, J. D. Bernal and R. H. Fowler successfully analyzed early X-ray diffraction data on water in terms of a disordered quartz-like structure, and concluded that the unique properties of water are due to the tetrahedral geometry. Since then, a number of experimental and theoretical studies have been published. Nevertheless, scientists have not yet captured a clear picture of liquid water. The debate is far from settled. Very recently, an international collaborative team led by Dr. A. Nilsson (Stanford Synchrotron Radiation Laboratory) and Professor S. Shin (RIKEN & The University of Tokyo) succeeded in obtaining X-ray spectroscopic evidence to support Röntgen's mixture model. Thanks to the brilliant synchrotron beamline at the SPring-8, the research group obtained some high resolution oxygen K-edge X-ray emission spectra of liquid water. The team found that there are two distinct narrow lone-pair derived peaks assigned, respectively, to tetrahedral and strongly distorted hydrogen-bonded species. For more information, see the paper, "High resolution X-ray emission spectroscopy of liquid water: The observation of two structural motifs", T. Tokushima et al., Chem. Phys. Lett., 460, 387-400 (2008).