Abstract
This paper analyzes the structural, energetic and mechanical properties of carbon dioxide hydrate clathrates calculated using finite cluster and periodic ab initio density-functional theory methodologies. Intermolecular interactions are described by the exchange-hole dipole moment method. The stability, gas saturation energetics, guest–host interactions, cage deformations, vibrational frequencies, and equation of state parameters for the low-pressure sI cubic phase of the CO2@H2O clathrate hydrate are presented. Our results reveal that: (i) the gas saturation process energetically favors complete filling; (ii) carbon dioxide molecules prefer to occupy the larger of the two cages in the sI structure; (iii) blue shifts occur in both the symmetric and antisymmetric stretching frequencies of CO2 upon encapsulation; and (iv) free rotation of guest molecules is restricted to a plane parallel to the hexagonal faces of the large cages. In addition, we calculate the librational frequency of the hindered rotation of the guest molecule in the plane perpendicular to the hexagonal faces. Our calculated spectroscopic data can be used as signatures for the detection of clathrate hydrates in planetary environments.
Highlights
Clathrate hydrates are of primary importance in a variety of fields, from life sciences to planetology, and constitute a natural resource in the energy industry
It is worth mentioning that, in comparison with the empty clathrate, the guest molecule produces a slight reduction of the lattice parameter, which can be explained by the attraction between the guest molecules and the host lattice
Repulsive forces play a decisive role in the stabilization of the clathrate structure by preventing its collapse, the attractive character of the guest–host interaction at the equilibrium geometry is supported by the reduced density gradient analysis discussed below
Summary
Clathrate hydrates are of primary importance in a variety of fields, from life sciences to planetology, and constitute a natural resource in the energy industry. The formation of clathrates has been proposed as a method to effectively store greenhouse gases by injecting volatiles into potential deep hydrate-forming deposits [1,2]. These crystalline compounds accommodate guest molecules (usually a non-polar gas) within the framework of a host three-dimensional network of water cages or channels. It depends on the nature of the guest molecule, clathrate hydrates generally need high pressure and low temperature to be stable. Giant moons, such as Ganymede or Titan, show evidence of having deep internal water-rich layers in several phases, including liquid water layers [3]
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