Effect of Guest–Host Hydrogen Bonding on Thermodynamic Stability of Clathrate Hydrates: Diazine Isomers

Guest–host
hydrogen bonding strongly affects the physical properties of clathrate
hydrate, such as the thermodynamic stability, water dynamics, and
dielectric properties, but attempts to quantify the effects of hydrogen
bonding on these properties are rare thus far. As a preliminary work,
this study investigates methane clathrate hydrates with three diazine
isomers, pyrazine, pyrimidine, and pyridazine, which expect nearly
the same van der Waals volumes due to their similar molecular shapes
and sizes, and their guest–host hydrogen-bonding behaviors.
The crystal structures of all three
binary diazine + CH4 hydrate phases were identified as
a cubic Fd3̅m structure, including
diazine
molecules in the 51264 cavity, commonly termed
as structure II hydrate, by a high-resolution powder diffraction pattern
analysis. The phase equilibrium curves of their clathrate hydrates
were obtained by the P–T trajectory of the
hydrate formation and dissociation process, and the thermodynamic
stability trend was well-explained by the guest–host hydrogen
bonding behavior as evaluated by the molecular polarities,
proton affinities, and ring-breathing vibration frequencies of the
three diazine isomers obtained from Raman spectroscopy. This study
provides useful information that contributes to the realization of
the expansion of the thermodynamics of clathrate hydrates to include
guest–host
hydrogen-bonding interactions.