Ab Initio Studies on the Clathrate Hydrates of Some Nitrogen- and Sulfur-Containing Gases

Ab initio calculations are performed to investigate the host-guest interactions and multiple occupancies of some sulfur- (H₂S, CS₂) and nitrogen-containing (N₂, NO, and NH₃) molecules in dodecahedral, tetrakaidecahedral, and hexakaidecahedral water cages in this work. Five functionals in the framework of density functional theory are compared, and the M06-2X method appears to be the best to predict the binding energies as well as the geometries. Results show that N₂ and NO molecules are more stable in the 5¹²6⁴ cage, while NH₃ and H₂S prefer to stabilize in the 5¹²6² cage. This suggests that the sI hydrates of NH₃ and H₂S exhibit higher stability than the sII structures and that sII NO hydrate is more stable than sI NO hydrate. N₂ is found to be more stable in type II structure with single occupancy and to form type I hydrate with multiple occupancy, which is consistent with the experimental observations. As to the guest molecule CS₂, it may undergo severe structural deformation in the 5¹² and 5¹²6² cage. For multiple occupancies, the 5¹², 5¹²6², and 5¹²6⁴ water cages can trap up to two N₂ molecules, and the 5¹²6⁴ water cage can accommodate two H₂S molecules. This work is expected to provide new insight into the formation mechanism of clathrate hydrates for atmospherically important molecules.