Guest motion in tetrahydrofuran clathrate hydrate studied by deuteron nuclear magnetic resonance

Abstract

The guest dynamics in tetrahydrofuran (THF) clathrate hydrate $(\text{THF}\ensuremath{\cdot}17{\text{H}}_{2}\text{O})$ was studied using several deuteron nuclear magnetic resonance (NMR) techniques. At low temperatures the magnetization recovery proceeds in two steps. The weight of the faster contribution decreases with decreasing temperatures. This behavior is the signature of a dynamical effect. The two contributions cannot be ascribed separately to the metadeuteron and to the paradeuteron of the THF molecule. The thermal evolution of the NMR spectra was described semiquantitatively using a distorted octahedral reorientational jump model. Pseudorotation has no significant impact on the spectral width. The motional correlation times, measured using two-time stimulated-echo and spin-relaxation techniques, cover a dynamic range of nine decades. Four-time stimulated-echo measurements reveal a statically heterogeneous rotational motion of the guest molecules in the 16-faced polyhedral cages of the structure II lattice.