Effects of Cations on the Hydrogen Bond Network of Liquid Water: New Results from X-ray Absorption Spectroscopy of Liquid Microjets (original) (raw)
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Effects of Alkali Metal Halide Salts on the Hydrogen Bond Network of Liquid Water
The Journal of Physical Chemistry B, 2005
Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.
Cationic and Anionic Impact on the Electronic Structure of Liquid Water
The Journal of Physical Chemistry Letters, 2017
Hydration shells around ions are crucial for many fundamental biological and chemical processes. Their local physicochemical properties are quite different from those of bulk water and hard to probe experimentally. We address this problem by combining soft X-ray spectroscopy using a liquid jet and molecular dynamics (MD) simulations together with ab initio electronic structure calculations to elucidate the water−ion interaction in a MgCl 2 solution at the molecular level. Our results reveal that salt ions mainly affect the electronic properties of water molecules in close vicinity and that the oxygen K-edge X-ray emission spectrum of water molecules in the first solvation shell differs significantly from that of bulk water. Ion-specific effects are identified by fingerprint features in the water X-ray emission spectra. While Mg 2+ ions cause a bathochromic shift of the water lone pair orbital, the 3p orbital of the Cl − ions causes an additional peak in the water emission spectrum at around 528 eV.
Journal of Electron Spectroscopy and Related Phenomena, 2005
We report the soft-X-ray absorption and emission studies of NaCl, MgCl 2 , and AlCl 3 in water solutions. The influences of cations on the water molecular structure can be seen as the absorption threshold edge shifted to high energy in the X-ray absorption spectra; the mixing of molecular orbital in 3a 1 symmetry is reinforced as the intensity of 3a 1 is further reduced; and the 1b 1 -emission peak shows the broadening and shift differently for Na + , Mg 2+ , and Al 3+ water solutions, which indicates that the charge difference of the cations may not be the only playing role being responsible to the interactions between the cations and water molecules.
Nature of the Aqueous Hydroxide Ion Probed by X-ray Absorption Spectroscopy
The Journal of Physical Chemistry A, 2007
X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OHions. In addition to spectral changes explicitly due to absorption by solvated OHions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OHion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OHion exists with a predominately "hyper-coordinated" solvation shell and that the OHion does not readily donate hydrogen bonds to the surrounding water molecules.
Journal of Physical Chemistry A, 2005
We utilized X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS) in order to study the ion solvation effect on the bulk hydrogen bonding structure of water. The fine structures in the X-ray absorption spectra are sensitive to the local environment of the probed water molecule related to the hydrogen bond length and angles. By varying the concentration of ions, we can distinguish between contributions from water in the bulk and in the first solvation sphere. We show that the hydrogen bond network in bulk water, in terms of forming and breaking hydrogen bonds as detected by XAS/XRS, remains unchanged, and only the water molecules in the close vicinity to the ions are affected.
The Journal of Physical Chemistry B, 2017
Interaction between water molecules and alkali metal ions in aqueous salt solutions has been studied by the oxygen K-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. In the measurement of several alkali halide aqueous solutions with different alkali chlorides (Li, Na, and K) and different sodium halides (Cl, Br, and I), the pre-edge component arising from the hydration water molecules shows a blue shift in peak energy as strongly depending on cations but not on anions. In the temperature dependent measurement, the pre-edge component arising from water molecules beyond the first hydration shell shows the same behavior as that of pure liquid water. On the other hand, the pre-edge component arising from water molecules in the first hydration shell of Li + ions is not evidently dependent on the temperature, indicating that the hydration water molecules are more strongly bound with Li + ions than the other water molecules. These experimental results are supported by the results of radial distribution functions of the first hydration shell and their temperature dependence, evaluated by molecular dynamics simulations.
Journal of Physical Chemistry A, 2003
We have studied the chemical bonding of water in the first hydration sphere to transition-metal ions in aqueous solutions by using X-ray absorption spectroscopy (XAS) combined with density functional theory calculations (DFT). The experimental technique is for the first time applied to the study of the oxygen K-edge absorption of liquid water in the presence of dissolved ions. We successfully resolved the electronic structure of water in the first hydration sphere. Features in the oxygen 1s XAS pre-edge region in Cr 3+ and Fe 3+ solutions are interpreted as mixing between the molecular orbitals of water and the open d shell of the transition-metal ion. These features are absent for Al 3+ solutions. Effects on the electronic structure from anions in the first hydration sphere are also described.
Spectroscopic probing of local hydrogen-bonding structures in liquid water
Journal of Physics: Condensed Matter, 2002
The hydrogen bond (H-bond) in liquid water holds the key to its peculiar behavior, with implications for chemical, biological and geological processes. In liquid water, the dynamical motion of atoms at the picosecond time-scale causes the H-bonds to break and reform resulting in a statistical distribution of different coordinations for the water molecules. Water molecules in liquid and solid phases exhibit two types of O-H interactions: strong covalent O-H bonds within the water molecule, and relatively weak H-bonds between the molecules. In ice, each water molecule is tetrahedrally coordinated by four neighboring waters through H-bonds (2 H-bonds to the oxygen atom, and one to each hydrogen). Although liquid water primarily exhibits the same coordination environment, experimental (vibrational spectroscopy, neutron and X-ray diffraction) and theoretical (molecular dynamics) studies have predicted that liquid water should contain a fraction of water molecules with broken H-bonds.
Structural Dynamics, 2014
The effect of monovalent cations (Li þ , K þ , NH 4 þ , Na þ ) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion-and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.
X-Ray Emission Spectroscopy of Hydrogen Bonding and Electronic Structure of Liquid Water
Physical Review Letters, 2002
We use x-ray emission spectroscopy to examine the influence of the intermolecular interaction on the local electronic structure of liquid water. By comparing x-ray emission spectra of the water molecule and liquid water, we find a strong involvement of the a 1 -symmetry valence-orbital in the hydrogen bonding. The local electronic structure of water molecules, where one hydrogen bond is broken at the hydrogen site, is separately determined. Our results provide an illustration of the important potential of x-ray emission spectroscopy for elucidating basic features of liquids.