How do natural gas hydrates form?

Ab initio geometry optimizations at MP2/6-31G*//HF/6-31G* level suggested that the CH₄(H₂O)₂₀ cluster with a CH₄ molecule within the (H₂O)₂₀ dodecahedral cavity has a stabilization energy of ~ 7 kcal/mole relative to separated CH₄ and (H₂O)₂₀ molecules. The cavity of a 20 mer fused cubic or edge-shared prismic structure was too small to enclose a CH₄ molecule. Even though the (H₂O)₂₁ cluster with H₂O molecule within the dodecahedral cavity was significantly more stable (~ 28 kcal/mole) than CH₄(H₂O)₂₀, the dodecahedral cage was too distorted in (H₂O)₂₁ to form a fused hydrate structure. The dodecahedral cage remained almost undistorted in CH₄(H₂O)₂₀ and hence, could form a fused hydrate structure. During a fused structure formation, each pentagonal ring shared by two dodecahedral structures or a dodecahedral and a tetrakaidecahedral structures resulted in stabilization by ~ 20-23 kcal/mole. Original is an abstract.