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Keywords:

  • methane hydrate;
  • clathrates;
  • dispersion-corrected density functional

Abstract

A search for stable local minima CH4 - (H2O)n closed methane clathrates was done at the ωB97X-D/aug-cc-pVDZ level of the theory, for n up to 20, starting from various different configurations for each size. The reliability of the method was validated by comparison to the complete basis set limit (CBSL) values of the methane–water interactions, of the water–water interactions and of the binding energies of (H2O)20 clusters. A potential model fitted to reproduce the CBSL interaction energy of the optimal CH4 - (H2O)n pair was also used in Monte Carlo optimizations. Confinement was found to occur already at n = 14. Optimizations with two empirical models for numerical simulations resulted in the same configurations. The addition of more water molecules, though, favored an increase of the size of the cage, instead of making an external hydrogen bond with the other water molecules in the cluster. An analysis was made at the ωB97X-D/aug-cc-pVDZ level of the interactions of methane with the confining clusters. In all cases, the interaction energy was negative, and for the dodecahedral cavity, the confinement of methane resulted in a significant stabilization relative to the unperturbed empty (H2O)20 cluster and an external methane molecule. Another analysis was made of the energetic cost of rotating the methane within the dodecahedral cavity. The corresponding barrier was lower than KBT at ambient temperature. © 2012 Wiley Periodicals, Inc.