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Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X-ray Scattering Yekkoni Lakshmanan Jeyachandran, Frank Meyer, Sankaranarayanan Nagarajan, Andreas Benkert, Marcus Baer, Monika Blum, Wanli Yang, Friedrich Reinert, Clemens Heske, Lothar Weinhardt, and Michael Zharnikov J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz502186a • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014

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The Journal of Physical Chemistry Letters

Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X-ray Scattering Yekkoni L. Jeyachandran,1,§ Frank Meyer,2 Sankaranarayanan Nagarajan,1,# Andreas Benkert,2,3 Marcus Bär,4,5,6 Monika Blum,6 Wanli Yang,7 Friedrich Reinert,2 Clemens Heske,3,6,8,9 Lothar Weinhardt,3,6,8,9* and Michael Zharnikov1* 1

Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

2

Experimentelle Physik VII, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

3

Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 4

Solar Energy Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 5

Institut für Physik und Chemie, Brandenburgische Technische Universität CottbusSenftenberg, Platz der Deutschen Einheit 1, 03046 Cottbus, Germany

6

Department of Chemistry, University of Nevada, Las Vegas (UNLV), 4505 Maryland Pkwy., Las Vegas, Nevada 89154-4003, USA 7

Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

8

ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology (KIT), Hermannv.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 9

Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18/20, 76128 Karlsruhe, Germany

§ #

Currently at Department of Physics, Bharathiar University, Coimbatore 641046, India

Currently at Department of Chemistry, National Institute of Technology, Tiruchirappalli – 620015, India

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Abstract

The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft x-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen K-edge emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure.

TOC graphic

Keywords: liquid water, ionic solvation, hydrogen-bond network, resonant inelastic soft xray scattering, x-ray emission spectroscopy, potassium chloride, proton dynamics


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Water is a seemingly simple molecule, consisting of two hydrogen (H) atoms covalently bonded to a single oxygen (O) atom that has two lone-pair electrons. The properties of free water molecules have been well-documented with x-ray and electron spectroscopies, both theoretically and experimentally.1-3 In contrast, water in the solid and, especially, in the liquid state is characterized by highly complex intermolecular structures that remain a subject of ongoing discussions.4-16 In the solid phase, a water molecule is coordinated with four neighboring molecules through hydrogen bonds (HBs), preferably in tetrahedral configuration. In the liquid phase, the HB configuration becomes transient and distorted, resulting, most likely, in lower coordinations of individual molecules.4,7 In the case of aqueous salt solutions, the HB structure of water is subject to further changes induced by ionic solvation.17,18 The understanding of the respective effects is of fundamental importance and significance for a variety of chemical and biological processes.18 Various approaches have been employed to investigate the molecular structure of liquid water, targeting, e.g., the local atomic arrangement by electron diffraction and x-ray or neutron scattering,10,19,20 as well as the vibrational characteristics and local electronic structure by infrared, Raman, photoelectron, and resonant Auger electron spectroscopy.7,10,21,22 Recently, these experiments have been complemented by synchrotron-based soft x-ray techniques, including x-ray absorption (XAS) and emission (XES) spectroscopies as well as resonant inelastic soft x-ray scattering (RIXS).3,4,6-8,11-14,22-25 These techniques provide comprehensive information on the electronic structure, giving insight into HB network and local configurations. In XAS, absorption resonances associated with unoccupied 4a1 and 2b2 orbitals are observed, with characteristic pre- (~534 eV), main- (~536 eV), and post-edge (~540 eV) spectral features interpreted as signatures of distorted and fully-coordinated HB configurations of water molecules, respectively.7,8 In XES, spectral features of both spectator and participant decay channels, associated with occupied orbitals and vibrational states, respectively, can be recorded. The spectator emission spectrum of liquid water comprises three distinct lines related to the occupied 1b2 (~521 eV), 3a1 (~525 eV), and 1b1 (~527 eV) molecular orbitals.6,9 The 1b2 emission is attributed to the internally bonding O 2p-H 1s orbital and is therefore very sensitive to changes in the internal O–H bonds.9 The 3a1 emission is ascribed to the mixed O 2s and O 2p O–H bonding orbital and is strongly affected by the external HB configuration.6,9 The 1b1 emission is associated with the non-bonding O2p lonepair orbital. This emission is expected to result in a strong single spectral line,6,9 which indeed is the case for gas phase water.1,3 However, high resolution measurements on ice deposits and liquid water show two lines in the spectral region of the 1b1 emission, around 526.0 eV (low 3 ACS Paragon Plus Environment


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energy, LE) and 526.8 eV (high energy, HE), with strong isotope and weak temperature effects.5,11,12 Competing interpretations of the HE and LE components have been published.11,12 On one hand, it is discussed that the XE spectra of water are a superposition of two independent contributions, where the HE component represents the 1b1 orbital in initial state HB configurations, while the LE peak is a final state signature of ultrafast molecular dissociation promoted by intact HBs of the probed H2O molecule.11,13 On the other hand, both HE and LE components are assigned to the 1b1 orbital in the initial state of water molecules, representative of the intact and strongly distorted HB configurations, respectively.12 Both interpretations have been thoroughly analyzed,13,14,23,24,26,27 and a recent study suggests that the dissociation of water molecules is indeed proton-transfer-mediated and energetically feasible in the presence of intact HBs15 (i.e., supporting the model of a superposition of initial and final state contributions)11. Note that intact HBs not only include four-fold, tetragonal configurations, as typically observed for ice Ih, but also configurations with a lower coordination number that have a similar geometry for individual HBs. These will also contribute to the LE line, but with a lower spectral weight than the configurations with fully intact HBs. In aqueous salt solutions, the local HB configurations of water molecules are expected to change due to their reorganization around ions in a hydration shell.18,28 This ion-promoted restructuring can be considered as an intermediate state between liquid and solid phases of water.29 Most of the previous soft x-ray studies on salt solutions were based on XAS measurements.28,30-33 In general, an increase in pre- and main-edge absorption intensities (associated with distorted HB structures) and a decrease in post-edge absorbance (related to intact HB configurations) were observed. These are indications of extensive structural reorganization, but it is difficult to gain more specific information on the character of local HB configurations based on XAS data alone. Useful additional information can be provided by XES, but only limited work has been performed on aqueous salt solutions so far.34,35 In the present work, aqueous potassium chloride (KCl) solutions of different concentrations were studied by collecting RIXS maps as well as resonant and non-resonant XE spectra at the O K-edge. RIXS maps are two-dimensional representations of the colorcoded emission intensity as a function of emission and excitation energy, providing comprehensive details of the electronic structure of a large variety of systems.3,13,25,36 The O K-edge RIXS maps of pure liquid water and three aqueous KCl solutions of different concentrations are shown in Fig. 1. These maps consist of the spectator emission region (i.e., below an emission energy of 529 eV) and the region around the Rayleigh line 4 ACS Paragon Plus Environment

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(i.e., the participant regime), which is shown on a magnified scale. Note that the term 'participant' describes here a process corresponding to the deexitation of the resonantly excited electron itself (elastic scattering), accompanied frequently by vibrational losses. The maps are dominated by the expected characteristic absorption and emission features of water molecules, pertaining to the unoccupied pre-edge (4a1) and main-edge (2b2) orbitals (labeled in red), as well as the occupied 1b2 and 3a1 orbitals and the LE/HE double peak structure at the position of the 1b1 orbital (labeled in black).3,13

Figure 1. O K-edge RIXS maps of pure liquid water (left) and aqueous KCl solutions of different concentrations (1, 2, and 3 M). The emission intensity is normalized to its maximum, color-coded (see the intensity scale bar), and shown as a function of emission and excitation energies. In the water map, the orbitals associated with the emission final states and absorption resonances are labeled in black and red, respectively. Distinct emission features are labeled A to D. The left and right parts of each map correspond to the spectator emission and the participant decay (intensity multiplied by a factor of 20) regimes, respectively. The maps of the KCl solutions exhibit noticeable differences from that of water, changing progressively with increasing salt concentration. Most easily seen is a shift of the absorption onset to higher excitation energies; while it lies at ~533.5 eV for pure water, it shifts successively to ~533.8, ~534.0, and ~534.1 eV for the 1, 2, and 3 M solutions, respectively (see the Supporting Information for further details). In the participant decay region (above an emission energy of 529 eV), these changes involve the spectral shape and relative intensities of the strong pre-edge resonance (marked A) and the weak main-edge resonance around 536.2 eV (marked B) of the Rayleigh line, as well as the energy loss tail (marked C) associated with vibrational excitations.13,16,37 These features are discussed in detail below after the following analysis of the spectator emission region. This analysis was performed based on separately measured XE spectra being identical but having a better statistics than the extracts from the RIXS maps. The non-resonant spectra of KCl solutions (1 M to 6 M) in Fig. 2 (left) show the distinct spectral changes. A close look at 5 ACS Paragon Plus Environment


the spectra reveals that the relative intensity of the LE line decreases and that of the 1b2 and 3a1 emissions increases with increasing salt concentration. These changes can be quantified by describing the spectra as a superposition of several independent contributions. We start by analyzing the spectrum of pure water. In agreement with previous results,11 for a satisfactory description, it requires two independent contributions, d1 and d2 (Fig. 2, right), which are representative of the initial state HB configurations and ultrafast dissociation on the time scale of the O 1s core hole decay (~3.6 fs)38, respectively. The component spectra were again derived by computing weighted differences between the spectra of H2O and D2O, such that the difference (component) spectra are positive at all energies. In accordance with the assignment as intact and dissociated water molecules, the d1 and d2 component spectra are very similar to the experimental XE spectra of gas phase water and sodium hydroxide solutions, respectively.3,11,13

XES - 550 eV KCl Normalized Intensity (a.u.)

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1b2

6M

d d2 Rd 3a1

Components 1

1b1

dR

4M d2 = H2O − 0.68D2O 3M d1 = D2O − 0.95H2O 2M

1M D2 O H2 O

H2O 520

522

524

526

528

520

522

524

526

528

Emission energy (eV)


Figure 2. Left: non-resonant O K-edge XE spectra of pure liquid water and aqueous KCl solutions of different concentrations (black). The spectra are normalized to the maximum of the HE line. Individual emission features are labeled. The spectra are fitted with three components shown as shaded areas and color-coded according to the right panel. The resulting fit is shown as a red line and the fit residuals are plotted at the bottom of the respective spectra. Right: O K-edge XE spectra of H2O and D2O, components d1 and d2, and residual component dR. d1 and d2 were derived from differences of the H2O and D2O spectra and dR from a difference of the salt spectra and the two components (d1 and d2) fit (see text for details). The analogous plots for pre- and main-edge excitation are presented in the Supporting Information (Fig. S2).


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To satisfactorily fit the spectra of the KCl solutions, a third, residual component (dR) had to be introduced, shown in the right panel of Fig. 2. It was derived by subtracting the weighted d1 and d2 spectra from each salt spectrum. Significantly, the same dR component was found to be suitable for all KCl concentrations, suggesting a true physical meaning of this component. Note that it differs from the spectra obtained by simply subtracting salt-solution spectra with different concentrations from each other. As shown in the Supporting Information (Fig. S3), this approach does not produce an adequate spectral component, at least not for the non-resonant case. Using the three components (dR, d1, and d2), an adequate fit to the XE spectra of all salt solutions could be obtained (Fig. 2, left), with the spectral weights of these components being the only (three) fitting parameters. An analogous decomposition could be performed for the cases of pre- and main-edge excitation as well (see the Supporting Information, Fig. S2). 56

16

XES dR / (d1+d2+dR) %

d2 / (d1+d2+dR) %

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52 48 44 40

12 8 4 0

0

1

2

3

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6

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Figure 3. Relative weights of the d2 (left) and dR (right) components, derived from the threecomponent analysis of the non-resonant XE spectra excited at 550 eV (Fig. 2), as functions of the salt concentration. The error bars represent the fitting errors. Fig. 3 shows the changes in the relative weights of the d2 and dR components (derived from the fits of the non-resonant spectra) as a function of the KCl concentration. Between the 0 and 6 M solutions, we find a decrease of d2 and an increase of dR by approx. −11 ± 2 % and +15 ± 3 %, respectively. These two values are similar, and the trend of the relative weights of the d2 and dR components correlates well with the composition ratio between the ions and water molecules (see the Supporting Information, Fig. S4). The spectrum of the dR component (Fig. 2, right) is interpreted as a representative of the specific coordination structure(s) of water molecules around the salt ions. This spectrum features three main peaks in the 1b1, 3a1, and 1b2 emission regions (similar to the d1 component associated with the initial state HB configurations), along with a shoulder at ~528.4 eV. Compared to the d1 component, we find a blue shift of the 1b2 peak and a red shift 7 ACS Paragon Plus Environment


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of the 1b1 peak, while the position of the 3a1 feature is almost unchanged. Thus, the 1b2 and 1b1 orbitals seem to be especially affected by the ion-induced reorganization of the HB network. According to calculations by Gråsjö et al.,39 the shift of the 1b2 peak towards the 1b1 emission might be attributed to a confinement of the water molecules due to the interaction with ions. In accordance with photoelectron spectroscopy experiments,39,40 we attribute the weak shoulder at ~528.4 eV to an interaction of water molecules with Cl- ions, leading to contributions from Cl 3p derived states. The dR component seems to appear predominantly at the expense of the d2 component, assigned to the ultrafast dissociation process promoted by intact HB configurations.11 This suggests that the hydration of KCl salt ions is likely associated with a different, specific configuration of water molecules that leads to a non- or less-dissociative core excited state, which correlates well with the previously reported XAS data for aqueous salt solutions.28,30-33 Furthermore, our XES-based analysis shows that the ion-induced distorted HB configurations differ from the distorted HB configurations in pure liquid water. It demonstrates the power of XES that a spectral signature (dR) of this new configuration can be derived. Note that the water molecules giving rise to the dR component still might be affected by a certain (lower) degree of ultrafast dissociation. The signature of this process will be mainly contained in the d2 component of the analysis. The conclusion regarding the mitigating effect of ions on the ultrafast dissociation is additionally supported by the participant decay spectra of pure liquid water and the aqueous KCl solutions. Such spectra for the pre-edge (534.2 eV) and main-edge (536 eV) excitations are shown in Fig. 4. They exhibit two pronounced emission components (I and II), as well as a tail towards lower emission energies for both the pre- and main-edge case. For pure water, the spectral weight lies on the low-energy emission (II), whereas, for the KCl solutions, it shifts gradually towards the high-energy emission (I) with increasing salt concentration. The participant decay spectra represent the process illustrated in the right panel of Fig. 4, where a water molecule is resonantly excited from the vibronic and electronic ground state to a strongly dissociative core-hole state.16,37 Subsequently, the wave packet representing the hydrogen atom follows the dissociative potential, with the molecule capable to decay back to a vibronic ground or excited state of the electronic ground state. The process is governed by the shape of the excited state potential curve and the lifetime (~3.6 fs for O 1s)38 of the coreexcited state. For emission that takes place before the wave packet moves considerably (blue), the system will decay back into the vibrational ground state, giving rise to feature I which therefore can be regarded as the "true elastic scattering" peak. For later emission processes, 8 ACS Paragon Plus Environment

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the wave packet progresses (green), leading to the excitation of higher vibronic levels in the emission process and giving rise to feature II which therefore can be regarded as quasi-elastic peak, corresponding to vibrational losses (the energy gap between components I and II, ~0.4 eV, is indeed close to the energy of the OH stretch in liquid water, ~0.42 eV41). Eventually, the wave packet might move far enough before the emission takes place, resulting in molecular dissociation with the respective signature, which is manifested as an extended tail in the participant emission spectrum (see the Supporting Information, Fig. S5).3,13 The spectral weights of this tail, as well as of feature II, for both pre- and main-edge excitation are thus a measure of how rapid the wave packet moves in the core-excited state. In this context, the observed gradual shift of the spectral weight from component II towards component I with increasing salt concentration (Fig. 4 left and center) has a physical meaning. It suggests that the dissociation probability of H2O molecules decreases in the salt solutions, in accordance with the above-given discussion of the spectator emission data. Interestingly, for all systems studied, this probability is higher for the 534.2 eV excitation as compared to that at 536 eV. This is related to the dissociative character of the 4a1 state, addressed at 534.2 eV.3

534.2 eV

I

536 eV

II

I II

H2O

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4M

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1b1

ground state

535

536 Internuclear distance


Figure 4. Participant decay spectra of pure liquid water and aqueous KCl solutions (1-6 M) for pre-edge (left) and main-edge (center) excitation. Right: schematic illustration of the nuclear wave packet dynamics on the time scale of the x-ray emission process upon pre-edge excitation. In summary, a reorganization of the HB network in aqueous KCl solutions of increasing concentration could be observed using in-situ O K RIXS maps and XE spectra at suitably selected excitation energies. This reorganization was shown to be profound, leading in 9 ACS Paragon Plus Environment


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particular to changes in the ultrafast molecular dissociation of the water molecules occurring at the timescale of the RIXS process. To derive these findings, spectator emission spectra were analyzed as a superposition of the ground state and dissociation component spectra, with the dissociation being promoted by intact HBs. For KCl solutions, a third XE component, associated with the hydration structure in the KCl solutions, was identified. As expected, the intensity of this component increases with increasing salt concentration, predominantly at the expense of the dissociation component. This suggests that the coordination structure(s) around the salt ions is/are distinctly different and less favorable for dissociation than the HB configurations of pure water. An additional support of this interpretation was provided by the analysis of the participant decay spectra, which reveal a strong vibronic coupling for both preedge (4a1) and main-edge (2b2) excitations. The changes observed in these spectra upon admixture of KCl to water could be reasonably explained by a gradual slow-down of the proton dynamics in the KCl solutions. The derived XE spectral signature of the hydration shell is of particular value as a reference, e.g., for theoretical simulations. With the ability to extract the specific signatures associated with ion solvation, similar characteristics can be obtained for other salt solutions and used to monitor the distinct effects of different cations and anions on the HB structure of water. This opens up opportunities of studying the complex hydration and interaction of biomolecules in salt solutions, as well as understanding the critical ion solvation/desolvation issues in modern electrochemical devices, e.g., fuel cells and batteries. These topics are grand challenges for both fundamental understanding and practical developments in biochemistry and sustainable energy applications.

Experimental Methods O K-edge RIXS maps as well as resonant and non-resonant XE spectra were collected with the SALSA (Solid and Liquid Spectroscopic Analysis) endstation36 at the undulator beamline 8.0.1 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The SALSA endstation is equipped with a custom-built flow-through liquid cell (total volume: 8.5 µl; "sample" volume: 0.05 µl) and a high-transmission soft x-ray spectrometer. The liquid flow rate in the cell is tuned such that the region accessible by the primary x-rays ("sample" volume) is renewed sufficiently often (~ 2000 Hz) to avoid beam damage and local heating effects. Ultra-pure water (H2O, Sigma Aldrich) and potassium chloride (KCl, Sigma Aldrich) were used to prepare aqueous KCl solutions with concentrations of 1, 2, 3, 4, and 6 mol/l. In the liquid cell these solutions were separated from the vacuum chamber by a window 10 ACS Paragon Plus Environment

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membrane of either silicon nitride (Si3N4, Silson, Inc.) or silicon carbide (SiC, NTT AT, Japan). All measurements were performed at room temperature (22 ºC). Additional details can be found in the Supporting Information.

Associated Content: Supporting Information: Additional details and experimental data as mentioned in the text. This information is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding Authors MZ: E-mail: [email protected]; LW: E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (projects No. ZH 63/16-1 and RE 1469/7-1). M. Bär acknowledges financial support by the Impuls- und Vernetzungsfonds of the Helmholtz-Association (VH-NG-423). The ALS is supported by the Department of Energy, Basic Energy Sciences, Contract No. DE-AC02-05CH11231.



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References (1) Nordgren, J.; Werme, L. O.; Agren, H.; Nordling, C.; Siegbahn, K. The X-ray Emission Spectrum of Water. J. Phys. B: Atom. Molec. Phys. 1975, 8, L18-L19. (2) Kashtanov, S.; Augustsson, A.; Luo, Y.; Guo, J.–H.; Sathe, C.; Rubensson, J.-E.; Siegbahn, H.; Nordgren, J.; Ågren, H. Local Structure of Liquid Water Studied by X-ray Emission Spectroscopy. Phys. Rev. B 2004, 69, 024201. (3) Weinhardt, L.; Benkert, A.; Meyer, F.; Blum, M.; Wilks, R. G.; Yang, W.; Bär, M.; Reinert, F.; Heske, C. Nuclear dynamics and Spectator Effects in Resonant Inelastic Soft Xray Scattering of Gas-Phase Water Molecules. J. Chem. Phys. 2012, 136, 144311. (4) Stillinger, F. H. Water Revisited. Science 1980, 209, 451-457. (5) Gilberg, E.; Hanus, M. J.; Foltz, B. Investigation of the Electronic Structure of Ice by High Resolution X-ray Spectroscopy. J. Chem. Phys. 1982, 76, 5093-5097. (6) Guo, J.-H.; Luo, Y.; Augustsson, A.; Rubensson, J.-E.; Såthe, C.; Ågren, H.; Siegbahn, H.; Nordgren, J. X-ray Emission Spectroscopy of Hydrogen Binding and Electronic Structure of Liquid Water. Phys. Rev. Lett. 2002, 89, 137402. (7) Wernet, Ph.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. Å.; Hirsch, T. K.; Ojamä, L.; Glatzel, P.; et. al. The Structure of the First Coordination Shell in Liquid Water. Science 2004, 304, 995-999. (8) Smith, J. D.; Cappa, C. D.; Wilson, K. R.; Messer, B. M.; Cohen, R. C.; Saykally, R. J. Energetics of Hydrogen Bond Network Rearrangements in Liquid Water. Science 2004, 306, 851-853. (9) Brena, B.; Nordlund, D.; Odelius, M.; Ogasawara, H.; Nilsson, A.; Pettersson, L. G. M. Ultrafast Molecular Dissociation of Water and Ice. Phys. Rev. Lett. 2004, 93, 148302. (10) Leetmaa, M.; Wikfeldt, K. T.; Ljungberg, M. P.; Odelius, M.; Swenson, J.; Nilsson, A.; Pettersson, L. G. M. Diffraction and IR/Raman Data do not Prove Tetrahedral Water. J. Chem. Phys. 2008, 129, 084502. (11) Fuchs, O.; Zharnikov, M.; Weinhardt, L.; Blum, M.; Weigand, M.; Zubavichus, Y.; Bär, M.; Maier, F.; Denlinger, J. D.; Heske, C; et al. Isotope and Temperature Effects in Liquid Water Probed by X-ray Absorption and Resonant X-ray Emission Spectroscopy. Phys. Rev. Lett. 2008, 100, 027801; 2008, 100, 249801 (Comment); 2008, 100, 249802 (Reply to Comment). (12) Tokushima, T.; Harada, Y.; Takahashi, O.; Senba, Y.; Ohashi, H.; Pettersson, L. G. M.; Nilsson, A.; Shin, S. High Resolution X-ray Emission Spectroscopy of Liquid Water: The Observation of Two Structural Motifs. Chem. Phys. Lett. 2008, 460, 387–400. 12 ACS Paragon Plus Environment

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