Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate

The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate was investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed a fast reorientation motion of water molecules in the temperature range of 212-278 K. The mean jump distance of hydrogen atoms was within 1.5-2.0A degrees. The relaxation time of water reorientation was estimated to be 100-410 ps with an activation energy of 10.2 +/- 5.8 kJ(.)mol(-1). The activation energy was in good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time of water reorientation is possibly due to strong interaction between a bromide anion and its surrounding water molecules (similar to so-called negative hydration), which suggests a unique strategy for designing efficient, safe, and inexpensive proton conductors having the framework of semiclathrate hydrates.