The infrared spectra
of sII gas hydrates have been computed using
density functional theory for the first time, at equilibrium, and
under pressure. It is also the first account of a full vibrational
analysis (both guest and host vibrations) for gas hydrates with hydrocarbon
guest molecules. Five hydrate structures were investigated: empty,
propane, isobutane, ethane–methane, and propane–methane
sII hydrates. The computed IR spectra are in good agreement with available
experimental and theoretical results. The OH stretching frequencies
were found to increase, while the H-bond stretching and H2O libration frequencies decreased with an increase in guest size
and cage occupancy and with a decrease in pressure. The H2O bending vibrations are relatively independent of guest size, cage
occupancy, pressure, temperature, and crystal structure. The guest
vibrational modes, especially the bending modes, also have minimal
pressure dependence. We have also provided more quantitative evidence
that gas hydrate material properties are defined by their hydrogen
bond properties, by linking H-bond strength to Young’s modulus.
The results and ensuing vibrational analysis presented in this paper
are a valuable contribution to the ongoing efforts into developing
more accurate gas hydrate identification and characterization methods
in the laboratory, in industry/nature, and even in outer space.