Hydrogen-Bond Spectroscopy of Water by Neutron Scattering

Abstract

We obtain a series of incoherent-inelastic-neutron-scattering spectra covering the complete frequency range of vibrations of hydrogen atoms in water over a temperature range of -15 to 80°C and also in ice at 20 K for reference. Comparison of these frequency spectra with those generated by computer molecular dynamics with flexible water molecules indicates that the proton motion in a free water molecule is substantially affected by formation of hydrogen bonds in the condensed phases.PACS numbers: 61.12.Fy, 35.20.Gs, 61.25.Em As a result of the existence of spin incoherence in neutron scattering from protons, neutrons are uniquely suited for probing the single-particle motions of protons in hydrogen-containing substances. This feature is particularly valuable for the case of water because of the fact that the hydrogen motion is sensitively affected by the formation of hydrogen bonds which is so characteristic of this substance. Although traditionally Raman scattering has been used extensively to study the hydrogenbond dynamics in water, 1 " 4 the quantitative theoretical prediction of the resulting Raman spectra of water has been difficult. This is due to the fact that the intensity of the Raman bands is related to the fluctuation spectra of the polarizability tensor of the molecules which is difficult to calculate when every molecule is interconnected to its neighbors by the hydrogen-bond network. 5 On the other hand, the incoherent neutron scattering spectrum singles out the vibrational and diffusive motions of the hydrogen atoms which can be directly simulated on a computer with molecular dynamics. 6 The analysis of the neutron spectrum in terms of a computer molecular dynamics (CMD) simulation serves the valuable purpose of refining the intramolecular and intermolecular potential model for water. 7 In this paper we report for the first time measurements of incoherent neutron scattering spectra of water extending to an energy transfer of 600 meV at reasonably small momentum transfer and at a range of temperatures including that of a supercooled state. 8 Such measurements have become possible because of the high intensities of epithermal neutrons produced by a pulsed spallation source. In parallel with the neutron experiments we have developed a new series of CMD simulations of water taking into account the internal vibrational motion of the molecule. Previous CMD studies of water used mainly potential models applicable to rigid molecules; thus the results give little information on the intramolecular part of the spectra. 6,9 The combination of experiment and simulation allows us to unambiguously identify and understand the features of the vibrational spectrum in the 50-500-meV range. We must qualify that although the neutron spectra are easier to interpret in terms of the proton motion, the Raman scattering is still superior in its energy resolution compared with the present stage of development of the neutron scattering technique.The experiment was carried out at the intense pulsed neutron source (IPNS) at t...

Light and neutron scattering studies of the OH stretching band in liquid and supercritical water

“…4 for the runs # 1, 2, 3, 4, 7: Also in this figure 2 2 , 1 and 3 , in the case of an isolated water molecule, are indicated by three vertical lines. We notice that the room temperature data are in excellent agreement with a previous experiment 12,13 performed using the HERMECS chopper spectrometer installed at the IPNS facility ͑Argonne National Laboratories, FIG. 2.…”

“…On the other hand inelastic neutron scattering studies in the same energy region have become feasible since the 80s, with the advent of spallation neutron sources, 11 and only the spectra of the normal and supercooled phases of H 2 O and D 2 O have been studied. 12,13 In the present paper we report a study of the O-H stretching spectra of water obtained by inelastic scattering of both light and neutrons, over a wide thermodynamic range, including supercritical states, as the comparison between these two techniques can shed some light over the interpretation of the complex Raman spectral shapes.…”

X-ray and Neutron Scattering of Water

“…This latter band shifts to a lower energy value of 217 meV in supercooled water, due to the formation of a more stable hydrogen bond network between neighboring molecules which makes a softer intramolecular attractive bond 158 . In the spectra also two bumps at 280 meV and 520 meV appear, which are the combination bands between the bending and the librational modes and between the stretching and the librational modes, respectively 142 .…”

Water adsorption on rutile TiO2(110) for applications in solar hydrogen production: A systematic hybrid-exchange density functional study

“…In this Appendix, the geometry and the vibrational frequencies of the isolated water molecule are reported and compared with previous studies [87][88][89][90][91][92][93] in order to assess the quality of the computational methodology adopted. The water molecule belongs to the C 2v point group.…”