Structure and dynamics of water and ions in solution Struktur und Dynamik von Wasser und Ionen in Lösung Weiche Röntgenabsorptions und emissionsuntersuchungen

Spectroscopies using soft X ray photons allow probing the valence and the unoccupied molecular orbitals of light elements e.g., carbon, nitrogen, oxygen through the K edge and of heavier elements like transition metals e.g., iron, cobalt, nickel through the L edge. As part of this thesis the first membrane free synchrotron based soft X ray absorption XA and high resolution X ray emission XE spectrometer was built for the investigation liquid samples delivered by a liquid micro jet. The Rowland circle based spectrometer can cover an emission energy range from 20 eV to 1000 eV. The first proof of principle spectra were obtained from pure water and aqueous Ni2 ions. Hydrogen bonding in liquid water was systematically studied by varying the number of hydrogen bonds per water molecule upon dilution in organic solvents. Using Fourier transform infrared spectroscopy, evidence was provided for the mixing ratio required for obtaining isolated water molecules in different oxygen free solvents. This information was used for the further X ray studies, where acetonitrile was selected as solvent, since it is a pure hydrogen bond acceptor and completely mixable with water. Oxygen K edge XA and XE spectra of water acetonitrile mixtures were obtained from a micro jet. The XA spectrum of pure liquid water shows the characteristic pre , main and post edge structure. Upon decreasing water concentration a strong relative increase and sharpening of the pre and main edge features is found. For the XE spectra of pure liquid water a vivid debate prevails concerning the physical origin of an additional spectral feature, which is not observed in the gas phase. Upon decreasing the water concentration, a relative attenuation of this additional feature is observed. The XA and XE spectral changes can be unambiguously related to the increasing amount of broken hydrogen bonds, and the hydrogen bond network of liquid water can thus be addressed on purely experimental grounds. The effect of nuclear dynamics on the absorption and emission spectra of the water acetonitrile mixtures was investigated by substituting H2O with D2O. It has a clearly distinguishable spectral fingerprint compared to the effect of hydrogen bonding. The special role of water in ionic solutions was investigated by studies of sodium ions dissolved in water, alcohol, and the respective mixed solvents. X ray absorption spectroscopy on the Na K edge revealed a shared solvation of the sodium ions in the mixed solvents. Specifically the water component is found to play a key role in stabilizing the solvation shell in these mixed solvent