Journal of Physical Chemistry A, Vol.105, No.31, 7429-7434, 2001
Stabilization of hydrate structure H by N-2 and CH4 molecules in 4(3)5(6)6(3) and 5(12) cavities, and fused structure formation with 5(12)6(8) cage: A theoretical study
At the MP2/6-31G*//HF/6-31G* level calculation, a dodecahedral water cluster (5(12), 20-mer) in hydrate structure H (sH) is more stable than an irregular dodecahedral cluster (4(3)5(6)6(3), 20-mer) by about 3.0 kcal/mol. However, in the presence of CH4 and N-2 molecules in cavity, the irregular dodecahedron becomes significantly more stable than a dodecahedral cluster. Even though a large 5(12)6(8) water cluster (36-mer) in sH is less stable (stabilization energy per H2O) than a tetrakaidecahedral (5(12)6(2), 24-mer, hydrate I) or a hexakaidecahedral (5(12)6(4), 28-mer, hydrate II) cluster, significant stabilization in sH is achieved by fused structure formation involving 5(12)6(8) clusters. While maximum stabilization is achieved by a pentagonal ring sharing between 5(12)6(8) and 5(12) cages, no stabilization is achieved by a pentagonal ring sharing between two 4(3)5(6)6(3) cages. Possible mechanism of hydrate formation has also been discussed.