Infrared Spectra of Methane Gas Hydrates from First-Principles

Methane gas hydrates are crystalline solids containing methane molecules enclosed in geometrical cage-like structures of water molecules. Presenting both a threat to environmental stability and an opportunity for climate change mitigation, methane gas hydrates require further study prior to practical attempts to address these issues. This study utilized density functional theory to simulate the infrared (IR) spectrum of structure I methane gas hydrates to characterize structure, chemical composition, and mechanical properties. The simulations were performed for various methane hydrate cage occupancies and for nonencaged methane in order to study the interactions of guest (methane) and host (water) molecules and to investigate potential methods of determining hydrate structure type and cage occupancy through IR spectroscopy. The frequency of the hydrogen bond stretch in the resulting spectra was additionally used to estimate Young’s modulus of the hydrates at varying cage occupancies. This is the first in-silico study of the IR spectra of sI methane hydrates with small (512) and large (51262) cages alternately occupied. All expected vibrational and librational modes were observed in the simulated infrared spectra. It was found that the vibrational frequencies of the methane molecules shifted to higher frequencies when encaged in the hydrate structure. The intensity of the C–H bend of methane in the infrared spectra relative to the intensity of other peaks was related to overall cage occupancy. The spectra exhibited two different peaks for the C–H asymmetrical stretch of methane, originating from methane molecules in small and large hydrate cages. The relative height of these peaks is an indicator of the ratio of small to large cage occupancy. Young’s modulus was found to increase with cage occupancy, indicating a stiffer structure at higher occupancy. The predicted correlations between structure, composition, and elasticity provide an efficient tool for gas hydrate material science, as well as for technological applications requiring the conversion of spectral signals into chemical and mechanical data.