Infrared Spectra of Gas Hydrates from First-Principles

Abstract

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.

From Infrared Spectra to Macroscopic Mechanical Properties of sH Gas Hydrates through Atomistic Calculations

“…These observations agree with the findings of Vlasic et al [20] for IR vibrational frequencies of sII gas hydrates. A good agreement is observed between the frequencies of empty and CH 4 -NH sH hydrates (this work) and empty and CH 4 -C 3 H 8 sII hydrates [20]. The intensity of vibrational peaks is a distinguishing feature of IR spectra.…”

“…On the other hand, sH hydrate filling causes an increase in the peak frequencies of the OH symmetric and asymmetric stretches, compared to empty sH. These observations agree with the findings of Vlasic et al [20] for IR vibrational frequencies of sII gas hydrates. A good agreement is observed between the frequencies of empty and CH 4 -NH sH hydrates (this work) and empty and CH 4 -C 3 H 8 sII hydrates [20].…”

“…This shift in spectral band peaks of water molecules is due to increased hydrogen bonding in hydrates. This shift has been previously confirmed for ice [31,32], gas hydrates [20], and other materials [33,34]. The OH stretching frequencies of water can experience significant redshifts as water condenses in structure.…”

“…A DFT study [38] showed that the low frequency vibrational modes of ice (Ih) are in good agreement with those obtained for sH gas hydrates, with some differences related to structures and simulation parameters. Vlasic et al [20] compiled some literature findings on vibrational frequencies of different structures of gas hydrates and ice.…”

“…Hiratsuka et al [13] used simulations of molecular dynamics to study the molecular vibrations of methane encapsulated in sI gas hydrate and showed that vibrations differ by cage type and are related to the C-H bond length. Vlasic et al [20] investigated the vibrational properties of sII gas hydrates using ab initio DFT. Experimental IR studies of gas hydrates were undertaken by Kumar et al [21] to identify the vibrations of carbon dioxide in different hydrate cages to estimate cage occupancies.…”