Effect of guest-host hydrogen bonding on the structures and properties of clathrate hydrates.

To provide improved understanding of guest-host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single-crystal X-ray crystallography has been used to detect guest-host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert-butylamine (tBA) sII clathrate with H(2)S/Xe help gases and the pinacolone + H(2)S binary sH clathrate. X-ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 A from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 A from the closest water oxygen atom. These distances are compatible with guest-water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X-ray crystallographic observations. The tBA guest shows long-lived guest-host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest-host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100-250 K. Guest-host hydrogen bonding leads to the formation of Bjerrum L-defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen-bonded guest-host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.