Thursday, January 28, 2016

Janda and Martin Groups demonstrated their existence for ring ether guests at T>200 K.


In clathrate hydrates van der Waals forces dominate cage-guest interactions at low temperatures because the hydrogen bonding propensity of the water is fully satisfied by the cage structure. However, the water molecules become more labile above 200 K and may interact strongly with the guests through hydrogen bonding. In a recent work published in The Journal of Physical Chemistry B, the Janda and Martin Groups compared the dynamics of tetrahydrofuran (THF) and cyclopentane (CP) guests in the hydrate cages above 200 K. Since THF can form hydrogen bonds while CP cannot, the dynamics provide insight into host-guest hydrogen bonding. Using sophisticated Magic Angle Spinning (MAS) solid-state NMR techniques, they measured proton spin-lattice relaxation times (T1) of the guests as a function of temperature, and found that the activation barrier to the motion of THF molecules is 4.7 kcal/mole (19.7 kJ/mole) at temperatures above 200 K. This is almost five times higher than the barrier at lower temperatures. In contrast, the barrier to guest motion in CP hydrate was found to be about 0.67 kcal/mole (2.8 kJ/mole), which agrees well with data at lower temperatures. These results demonstrate that strong guest-host hydrogen bonding exist between the THF guest and the clathrate cage above 200 K due to the host lattice mobility.

Read the paper: . J. Phys. Chem B, 2015, 119, 15485 – 15492. DOI: 10.1021/acs.jpcb.5b08369