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 1.5–2.0 Å. 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.