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Title: Nature of the Aqueous Hydroxide Ion Probed by X-ray AbsorptionSpectroscopy

Abstract

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 OH{sup -} ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, 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 OH{sup -} ion 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 ismore » 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 OH{sup -} ion exists with a predominately 'hyper-coordinated' solvation shell and that the OH{sup -} ion does not readily donate hydrogen bonds to the surrounding water molecules.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
Sponsoring Org.:
USDOE Director. Office of Science. Basic EnergySciences
OSTI Identifier:
917054
Report Number(s):
LBNL-62752; #341
Journal ID: ISSN 1089-5639; JPCAFH; R&D Project: 403501; BnR: KC0301020; TRN: US0804419
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry A; Journal Volume: 111; Related Information: Journal Publication Date: 2007
Country of Publication:
United States
Language:
English
Subject:
75; ABSORPTION; ABSORPTION SPECTRA; ABSORPTION SPECTROSCOPY; ELECTRIC FIELDS; HYDROGEN; HYDROXIDES; POTASSIUM HYDROXIDES; SPECTRA; WATER

Citation Formats

Cappa, Christopher D., Smith, Jared D., Messer, Benjamin M., Cohen, Ronald C., and Saykally, Richard J. Nature of the Aqueous Hydroxide Ion Probed by X-ray AbsorptionSpectroscopy. United States: N. p., 2007. Web. doi:10.1021/jp070551c.
Cappa, Christopher D., Smith, Jared D., Messer, Benjamin M., Cohen, Ronald C., & Saykally, Richard J. Nature of the Aqueous Hydroxide Ion Probed by X-ray AbsorptionSpectroscopy. United States. doi:10.1021/jp070551c.
Cappa, Christopher D., Smith, Jared D., Messer, Benjamin M., Cohen, Ronald C., and Saykally, Richard J. Sun . "Nature of the Aqueous Hydroxide Ion Probed by X-ray AbsorptionSpectroscopy". United States. doi:10.1021/jp070551c.
@article{osti_917054,
title = {Nature of the Aqueous Hydroxide Ion Probed by X-ray AbsorptionSpectroscopy},
author = {Cappa, Christopher D. and Smith, Jared D. and Messer, Benjamin M. and Cohen, Ronald C. and Saykally, Richard J.},
abstractNote = {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 OH{sup -} ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, 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 OH{sup -} ion 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 OH{sup -} ion exists with a predominately 'hyper-coordinated' solvation shell and that the OH{sup -} ion does not readily donate hydrogen bonds to the surrounding water molecules.},
doi = {10.1021/jp070551c},
journal = {Journal of Physical Chemistry A},
number = ,
volume = 111,
place = {United States},
year = {Sun Mar 04 00:00:00 EST 2007},
month = {Sun Mar 04 00:00:00 EST 2007}
}
  • The electronic structure of aqueous NH{sub 3} and ND{sub 3} has been investigated using resonant inelastic soft x-ray scattering. Spectral features of different processes involving nuclear dynamics in the core-excited state can be identified. When exciting into the lowest core-excited state, we find a strong isotope effect and clear evidence for ultrafast proton dynamics. Furthermore, a strong vibronic coupling is observed and, in the case of aqueous NH{sub 3}, a vibrational fine structure can be resolved.
  • We employed ultrafast transient absorption and broadband 2DIR spectroscopy to study the vibrational dynamics of aqueous hydroxide solutions by exciting the O–H stretch vibrations of the strongly hydrogen-bonded hydroxide solvation shell water and probing the continuum absorption of the solvated ion between 1500 and 3800 cm{sup −1}. We observe rapid vibrational relaxation processes on 150–250 fs time scales across the entire probed spectral region as well as slower vibrational dynamics on 1–2 ps time scales. Furthermore, the O–H stretch excitation loses its frequency memory in 180 fs, and vibrational energy exchange between bulk-like water vibrations and hydroxide-associated water vibrations occursmore » in ∼200 fs. The fast dynamics in this system originate in strong nonlinear coupling between intra- and intermolecular vibrations and are explained in terms of non-adiabatic vibrational relaxation. These measurements indicate that the vibrational dynamics of the aqueous hydroxide complex are faster than the time scales reported for long-range transport of protons in aqueous hydroxide solutions.« less
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  • The effect of isovalent chemical substitution on the magnitude and coherence length of local ferroelectric distortions present in sub-20 nm Ba 1–xSr xTiO 3 (x = 0.0, 0.30, 0.50, 1.0) and BaTi 1–yZr yO 3 (y = 0.0, 0.15, 0.50, 1.0) nanocrystals synthesized at room temperature is investigated using X-ray absorption near edge structure (XANES) and pair distribution function analysis of X-ray total scattering data (PDF). Although the average crystal structure of the nanocrystals is adequately described by a centrosymmetric, cubic Pm3¯m space group, local ferroelectric distortions due to the displacement of the titanium atom from the center of themore » perovskite lattice are observed for all compositions, except BaZrO 3. The symmetry of the ferroelectric distortions is adequately described by a tetragonal P4mm space group. The magnitude of the local displacements of the titanium atom in BaTiO 3 nanocrystals is comparable to that observed in single crystals and bulk ceramics, but the coherence length of their ferroelectric coupling is much shorter (≤20 Å). Substitution of Sr 2+ for Ba 2+ and of Zr 4+ for Ti 4+ induces a tetragonal-to-cubic transition of the room temperature local crystal structure, analogous to that observed for single crystals and bulk ceramics at similar compositions. This transition is driven by a reduction of the magnitude of the local displacements of the titanium atom and/or of the coherence length of their ferroelectric coupling. Replacing 50% of Ba 2+ with Sr 2+ slightly reduces the magnitude of the titanium displacement, but the coherence length is not affected. In contrast, replacing 15% of the ferroelectrically active Ti 4+ with Zr 4+ leads to a significant reduction of the coherence length. Deviations from the ideal solid solution behavior are observed in BaTi 1–yZr yO 3 nanocrystals and are attributed to an inhomogeneous distribution of the barium atoms in the nanocrystal. Composition–structure relationships derived for Ba 1–xSr xTiO 3 and BaTi 1–yZr yO 3 nanocrystals demonstrate that the evolution of the room temperature local crystal structure with chemical composition parallels that of single crystals and bulk ceramics, and that chemical control over ferroelectric distortions is possible in the sub-20 nm size range. In addition, the potential of PDF analysis of total scattering data to probe compositional fluctuations in nanocrystals is demonstrated.« less