The Electronic Structure of the Hydrated Proton:  A Comparative X-ray Absorption Study of Aqueous HCl and NaCl Solutions

Cappa, C. D.; Smith, J. D.; Messer, Benjamin; Cohen, R. C.; Saykally, Richard J.

doi:10.1021/jp0534582

Cited by 43 publications

(68 citation statements)

References 33 publications

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“…It is not universally agreed upon whether the Cl -ion itself imparts any specific changes to the features of the XAS of different solutions. 18,35,36 Our own dynamics calculations indicate that the first solvation shell of Cl -, identified by water molecules with one or more hydrogen oriented toward the anion, is floppy with rapid exchange events. The average coordination number is 5.7, and the overall distribution, primarily with coordination numbers between 4 and 7, is broad ( Figure 2).…”

Section: Resultsmentioning

confidence: 88%

“…Nevertheless, earlier results, also from Cappa et al, compared 4 M HCl and NaCl solutions and found the HCl spectrum to be very similar to that for bulk water and dissimilar from that of NaCl. 36 The authors suggested that the effects of H + and Cl -roughly counteracted each other, yielding spectroscopic features very much like those of bulk water. However, other studies have suggested 35 that the cations contribute more significantly to the features of the X-ray absorption spectrum because they more directly interact with the oxygens of water.…”

Section: Figurementioning

confidence: 99%

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Local Effects in the X-ray Absorption Spectrum of Salt Water

et al. 2010

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Both first-principles molecular dynamics and theoretical X-ray absorption spectroscopy have been used to investigate the aqueous solvation of cations in 0.5 M MgCl 2 , CaCl 2 , and NaCl solutions. We focus here on the species-specific effects that Mg 2+ , Ca 2+ , and Na + have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the hydrogen-bonding characteristics of the more rigid magnesium first-shell water molecules differ from those in the more flexible solvation shell surrounding calcium. In particular, the first solvation shell water molecules of calcium are able to form acceptor hydrogen bonds, and this results in an enhancement of a post-edge peak near 540 eV. The absence of acceptor hydrogen bonds for magnesium first shell water molecules provides an explanation for the experimental and theoretical observation of a lack of enhancement at the post-main-edge peak. For the sodium monovalent cation we find that the broad tilt angle distribution results in a broadening of postedge features, despite populations in donorand-acceptor configurations consistent with calcium. We also present the reaveraged spectra of the MgCl 2 , CaCl 2 , and NaCl solutions and show that trends apparent with increasing concentration (0.5, 2.0, 4.0 M) are consistent with experiment. Finally, we examine more closely both the effect that cation coordination number has on the hydrogen-bonding network and the relative perturbation strength of the cations on lone pair oxygen orbitals.

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Section: Resultsmentioning

confidence: 88%

Section: Figurementioning

confidence: 99%

Local Effects in the X-ray Absorption Spectrum of Salt Water

et al. 2010

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“…16,21,22 As a starting point, XA spectra of the bare OH -ion and of a free H 2 O molecule have been calculated ( Figure 2); the influence of solvation environment will be discussed shortly. These calculations indicate that the bare OH -ion absorbs at lower energies than do free H 2 O molecules, consistent with the experimental observations for the aqueous solutions.…”

Section: Discussionmentioning

confidence: 99%

Nature of the Aqueous Hydroxide Ion Probed by X-ray Absorption Spectroscopy

et al. 2007

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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 -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 -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 -ion exists with a predominately "hyper-coordinated" solvation shell and that the OH -ion does not readily donate hydrogen bonds to the surrounding water molecules.

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“…3,[9][10][11][12] The details, such as the depth profile of ion concentrations, depend very much on the molecular model and interaction potentials assumed. [9][10][11][12] Experiments carried out with scanning tunneling microscopy (STM) 13 , X-ray spectroscopy 14,15 , and attenuated total reflection (ATR) 8 have been used to verify the theoretical prediction. They however suffer from either the water surface being too dynamic or the techniques not being sufficiently surface-specific.…”

Section: Introductionmentioning

confidence: 99%

Interfacial Structures of Acidic and Basic Aqueous Solutions

et al. 2008

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Phase-sensitive sum-frequency vibrational spectroscopy was used to study water/vapor interfaces of HCl, HI, and NaOH solutions. The measured imaginary part of the surface spectral responses provided direct characterization of OH stretch vibrations and information about net polar orientations of water species contributing to different regions of the spectrum. We found clear evidence that hydronium ions prefer to emerge at interfaces. Their OH stretches contribute to the "ice-like" band in the spectrum. Their charges create a positive surface field that tends to reorient water molecules more loosely bonded to the topmost water layer with oxygen toward the interface, and thus enhances significantly the "liquid-like" band in the spectrum. Iodine ions in solution also like to appear at the interface and alter the positive surface field by forming a narrow double-charge layer with hydronium ions. In NaOH solution, the observed weak change of the "liquid-like" band and disappearance of the "ice-like" band in the spectrum indicates that OH-ions must also have excess at the interface. How they are incorporated in the interfacial water structure is however not clear.

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The Electronic Structure of the Hydrated Proton: A Comparative X-ray Absorption Study of Aqueous HCl and NaCl Solutions

Cited by 43 publications

References 33 publications

Local Effects in the X-ray Absorption Spectrum of Salt Water

Local Effects in the X-ray Absorption Spectrum of Salt Water

Nature of the Aqueous Hydroxide Ion Probed by X-ray Absorption Spectroscopy

Interfacial Structures of Acidic and Basic Aqueous Solutions

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