Water dynamics on the hydrate lattice of a tetrabutyl ammonium bromide semiclathrate

Deuteron nuclear magnetic resonance (NMR) and dielectric spectroscopy are utilized to investigate the dynamics of the water molecules in the semiclathrate (tetra-n-butyl ammonium bromide) 26 H(2)O. Stimulated-echo spectroscopy reveals a nonexponential correlation function predominantly due to rotational motion with jump angles that are broadly distributed around the tetrahedral angle. The reorientational correlation times from this technique agree excellently with those from dielectric measurements, both resulting in an activation energy of (43+/-1) kJ/mol. Large, spatially varying electrical dipolar fields, set up by the Br(-) and the N(+) ions located on the hydrate lattice, are held responsible for the pronounced stretching of the correlation functions. Solid-echo spectra were acquired over a broad temperature range. They exhibit an apparent two-phase character discussed in terms of various scenarios. Two-dimensional NMR spectra and four-time stimulated echoes were recorded, but an exchange of slow and fast subensembles could not be detected. Spin-lattice relaxation does not directly reflect the local reorientational motion and its nonexponentiality is interpreted with reference to the translational dynamics of the water molecules.