Molecular Dynamics Simulations of Guest – Host Hydrogen Bonding in Structure I, Ii, and H Clathrate Hydrates

The standard picture is that clathrate hydrates form when hydrophobic guest molecules are compressed with water under high pressure-low temperature conditions. In the ice-like hydrate framework, water molecules form molecule-sized cavities that encapsulate the guests and minimize water-hydrophobic guest interactions. This picture, however, must be modified by observations that many water soluble, hydrogen-bonding molecules form hydrates under relatively mild pressure-temperature conditions as well. Furthermore, these hydrates form with the same canonical structures as hydrophobic guests. We recently performed a series of molecular dynamics simulations, single crystal X-ray crystallographic structural determinations, and NMR relaxation time experiments to explicitly study the presence of hydrogen bonds between the guest and host in clathrate hydrates. Systems studied include binary structure H hydrates of tertbutylmethylether and pinacolone, the binary structure II hydrates of tetrahydrofuran, tetrahydropyran, tert-butylamine, 1-propanol, 2-propanol, and ethanol and structure I hydrates of ethanol. These guests form hydrogen bonds of varying stability and lifetime which affect the hydrate structure and the dynamics of both the water host and the guests. Spectroscopic signatures of the guest-host hydrogen bonding are briefly discussed.