Accurate crystal structures of ices from X-ray and ED with Hirshfeld atom refinement

Abstract

Background. Water is an essential chemical compound for living organisms, and twenty of its different crystal solid forms (ices) are known. Still, there are many fundamental problems with these structures such as establishing the correct positions and thermal motions of hydrogen atoms. The list of ice structures is not yet complete as DFT calculations and spectroscopic measurements have suggested existence for additional as of yet unknown phases. In many ice structures, neither neutron diffraction nor DFT calculations nor X - ray diffraction methods can easily solve the problem of hydrogen atom disorder or accurately determine their atomic displacement parameters. Methods. We applied a new way of refinement of single crystal high pressure X - ray synchrotron and laboratory X -ray and electron diffraction data called Hirshfeld Atom Refinement. This method utilizes aspherical atomic scattering factors (X - rays), and aspherical atomic electrostatic potentials (ED), based on so called stockholder (Hirshfeld) partition and is especially effective in the case of refinement of crystals of H -rich compounds. Results. Here we present accurate crystal structures of H2O, D2O and mixed (50%H2O/50%D2O) ice VI and ice VII obtained by Hirshfeld Atom Refinement (HAR) against high pressure single crystal synchrotron and laboratory X - ray diffraction data as well as results of refinement of hexagonal ice obtained by HAR against electron diffraction data. It was possible to obtain O - H bond lengths and anisotropic atomic displacement parameters for disordered hydrogen atoms which are in good agreement with the corresponding results of single crystal neutron diffraction data.[1] Conclusions. Our results show that Hirshfeld atom refinement against X - ray diffraction and electron diffraction data is a tool which can