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

Abstract

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.