Theoretical studies of CH_4(H_2O)_(20), (H_2O)_(21), (H_2O)_(20), and fused dodecahedral and tetrakaidecahedral structures: How do natural gas hydrates form?

摘要

Ab initio geometry optimizations (HF/6-31G~*) followed by single point energy calculations (MP2/6-31G~*) suggest that the CH_4(H_2O)_(20) cluster with a CH_4 molecule within the (H_2O)_(20) dodecahedral cavity has a stabilization energy (SE) of around 7 kcal/mol relative to separated CH_4 and (H_2O)_(20) molecules. The cavity of a 20 mer fused cubic or edge-shared prismic structure is too small to enclose a methane molecule. Even though the (H_2O)_(21) cluster with a water molecule within the dodecahedral cavity is significantly more stable (by around 28 kcal/mol) than CH_4(H_2O)_(20), the dodecahedral cage is too distorted in (H_2O)_(21) to form a fused hydrate structure. In CH_4(H_2O)_(20), on the other hand, the dodecahedral cage remains almost undistorted and hence, can form a fused hydrate structure. The present study also suggests that during a fused structure formation, each pentagonal ring sharing between two dodecahedral structures or a dodecahedral and a tatrakaidecahedral structures results in stabilization by around 20-23 kcal/mol.