Signatures in Vibrational Spectra of Ice Nanotubes Revealed by a Density Functional Tight Binding Method

The geometry structures and vibrational infrared and resonant Raman spectra of ordered n -gonal water nanotubes, n = 5−7, were systematically studied using a self-consistent charge density-functional tight-binding method complemented with an empirical van der Waals force correction. It is shown that water molecules can form cylindrical crystalline structures, referred to as ice nanotubes, by hydrogen bonding under confinement within single-walled carbon nanotubes. The hydrogen bond plays an important role not only in formation of the unique structures of ice nanotubes but also in the signatures in their vibrational spectra. The calculated infrared spectra in the low-frequency domain show a series of weak bands involving hydrogen bonds along with two librational bands which arise from the constraints induced by hydrogen bonding. In the middle-to-high-frequency region, the intramolecular bending bands of ice nanotubes are shifted to lower frequencies with regard to those of conventional hexagonal ice phase, whereas the intramolecular symmetric and asymmetric stretching bands are shifted to higher frequencies as the hydrogen-bond networks of ice nanotubes are weaker than those of conventional hexagonal ice. The predicted resonant Raman spectra also present distinctive features between those ice nanotubes.