Anomalous broadening of water intramolecular bending mode in ice-Ih

Vibrational dynamics of different ice phases and water in minerals have been widely studied by inelastic neutron scattering (INS). The observed intramolecular O-H stretching (390-460meV), and intermolecular translational (0-40meV) and librational (50-140meV) modes have been well described computationally. However, in ice-Ih the intramolecular bending mode peak (BMP, the “scissors mode”) H-O-H is very broad (from 175 to 235meV, FWHM=45meV), whereas that for water confined in minerals (e.g., beryl, cordierite, gypsum, bassanite, hemimorphite, mordenite) is much narrower (FWHM=4-11meV). In these minerals water either does not have hydrogen bonds (beryl, cordierite), or has ordered hydrogen bonds with the confinement. The BMP broadening in ice-Ih could be due to quantum effects but, if so, why do these quantum effects not affect the BMP for water in beryl, where water has been shown to exhibit quantum tunneling? Alternatively, this could be due to the proton-disordered structure of ice-Ih, but available calculations do not reproduce the BMP broadening. Recent INS spectra for D₂O ice-Ih, obtained to study the isotopic effect, and for proton-ordered H₂O ice-VIII, shed light on this question, and the results of these and parallel computer simulations will be discussed.