Theoretical studies of CH4(H2O)20, (H2O)21, (H2O)20, and fused dodecahedral and tetrakaidecahedral structures:: How do natural gas hydrates form?

Ab initio geometry optimizations (HF/6-31G(*)) followed by single point energy calculations (MP2/6-31G(*)) suggest that the CH4(H2O)(20) cluster with a CH4 molecule within the (H2O)(20) dodecahedral cavity has a stabilization energy (SE) of around 7 kcal/mol relative to separated CH4 and (H2O)(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 (H2O)(21) cluster with a water molecule within the dodecahedral cavity is significantly more stable (by around 28 kcal/mol) than CH4(H2O)(20), the dodecahedral cage is too distorted in (H2O)(21) to form a fused hydrate structure. In CH4(H2O)(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 tetrakaidecahedral structures results in stabilization by around 20-23 kcal/mol. (C) 1999 American Institute of Physics.