Stabilization of hydrate structure H by N₂ and CH₄ molecules in 4³5⁶6³ and 5¹² cavities, and fused structure formation with 5¹²6⁸ cage: A theoretical study

At the MP2/6-31G*//HF/6-31G* level calculation, a dodecahedral water cluster (5¹², 20-mer) in hydrate structure H (sH) is more stable than an irregular dodecahedral cluster (4³5⁶6³, 20-mer) by about 3.0 kcal/mol. However, in the presence of CH₄ and N₂ molecules in cavity, the irregular dodecahedron becomes significantly more stable than a dodecahedral cluster. Even though a large 5¹²6⁸ water cluster (36-mer) in sH is less stable (stabilization energy per H₂O) than a tetrakaidecahedral (5¹²6², 24-mer, hydrate I) or a hexakaidecahedral (5¹²6⁴, 28-mer, hydrate II) cluster, significant stabilization in sH is achieved by fused structure formation involving 5¹²6⁸ clusters. While maximum stabilization is achieved by a pentagonal ring sharing between 5¹²6⁸ and 5¹² cages, no stabilization is achieved by a pentagonal ring sharing between two 4³5⁶6³ cages. Possible mechanism of hydrate formation has also been discussed.