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

Abstract

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.