DFT-assisted interpretation of the Raman spectra of hydrogen-ordered ice XV

The vibrational spectra of the condensed phases of water often show broad and strongly overlapping spectral features which can make spectroscopic interpretations and peak assignments difficult. The Raman spectra of hydrogen-ordered H₂O and D₂O ice XV are reported here, and it is shown that the spectra can be fully interpreted in terms of assigning normal modes to the various spectral features by using density functional theory (DFT) calculations. The calculated lattice-vibration spectrum of the experimental antiferroelectric structure is in good agreement with the experimental data whereas the spectrum of a ferroelectric Cc structure, which computational studies have suggested as the crystal structure of ice XV, differs substantially. Moreover, the calculated coupled O–H stretch spectrum also seems in better agreement with the experiment than the calculated spectrum for the Cc structure. Both the hydrogen bonds as well as the covalent bonds appear to be stronger in hydrogen-ordered ice XV than in the hydrogen-disordered counterpart ice VI. A new type of stretching mode is identified, and it is speculated that this kind of mode might be relevant for other condensed water phases as well. Furthermore, the ice XV spectra are compared to the spectra of ice VIII which is the only other high-pressure phase of ice for which detailed spectroscopic assignments have been made so far. In summary, we have established a link between crystallographic data and spectroscopic information in the case of ice XV by using DFT-calculated spectra. Such correlations may eventually help interpreting the vibrational spectra of more structurally-disordered aqueous systems. Copyright © 2012 John Wiley & Sons, Ltd.