Computational analysis of vibrational spectrum and hydrogen bonds of ice XVII

Abstract

Based on first-principles density functional theory, we investigated the relationship between the vibrational normal modes and the spectrum of the newest laboratory-prepared ice phase, an empty clathrate hydrate structure from gas hydrate named ice XVII. A 48-molecule supercell was designed to mimic the hydrogen-disordered structure. Despite its much lower density than ice Ih, its phonon density of states shows features very similar to those of that phase. In our previous studies of ice Ic and ice XIV, we found two basic hydrogen bond vibrational modes in these hydrogen-ordered ice phases, which contribute two sharp hydrogen bond peaks in the translation region. In this study, we found that this rule also holds in the hydrogen-disordered phase ice XVII. A water molecule vibrating along its angle bisector possesses strong energy, because this vibrational mode involves oscillation against four bonded neighbors. In contrast, a water molecule vibrating perpendicular to its angle bisector has low energy because this mode involves only two of the molecule's hydrogen bonds. This is an evidence in hydrogen-disordered ice and strengthens our proposal that the existence of two basic hydrogen bond vibrational modes is a general rule among ice family. Figure 5. Smoothed probability density distribution of bond angles of ice XVII (blue) and ice Ih (black). Inset: Normal mode at 1647 cm −1 . This vibrational mode is mainly contributed by water molecules with large bond angles, which are highlighted in gold.

Computational Analysis of Exotic Molecular and Atomic Vibrations in Ice XV

“…We also found “rigid network” modes in ice VIII and VII [2] and “isolated stretching” modes in ice XIV [12]. According to our series of investigations of ice phases, we identified two intrinsic categories of molecule vibrations in the translational band that constitute two sharp peaks [2,12,13,14,15]. In this paper, we confirm the presence of cluster vibrations in a much lower region of the translation band.…”

Density of Phonon States in Cubic Ice Ic

“…From this plot, we can easily recognize two well-separated groups of bands relative to the rigid H 2 O molecule vibrations, that is, the lattice phonon modes, both acoustic and optic (ℏω < 45 meV), and the librational modes, that is, wagging, rocking, and twisting 27 (65 meV < ℏω < 130 meV). By looking at the former type of bands of both ice I structures, one can observe that the lattice phonon region of Z H (ω) is quite similar to what can be observed in other ice forms, even at high pressure, 21 but is different from the lattice bands pertaining to empty hydrate structures, such as ice XVI 33 and ice XVII, 34,35 that show a cleft of the main acoustic peak at about 7 meV. In addition, the librational bands show a quite different shape passing from the open and channeled structure of ice XVII to the cubic and hexagonal structure of ice I, with a more intense principal peak at about 70 meV and a deeper groove placed at 80 meV in ice XVII.…”

A Strategy for the Analysis of the Far-Infrared Vibrational Modes of Hydrogen-Disordered Ice V

“…This shows that high pressure deformed the tetrahedral structure of ice V. Thus, as the lattice mainly comprises four-molecule rings, this deformation leads to differentiated bond angles in ice V: the hydrogen bond angles which constitute the rings are 95 ± 5°, and the others are 125 ± 5°. Moreover, the H-bond lengths of ice V range from 1.81 to 1.93 Å, which is a broader distribution of H-bond lengths than that in ice Ic (1.79 to 1.87 Å) . As ice V and ice XIII have the same lattice, these phenomena are also seen in ice XIII.…”