Deuteron magnetic resonance and dielectric studies of guest reorientation and water dynamics in six clathrate hydrates containing ring-type guests

Abstract

Using deuteron nuclear magnetic resonance and high-resolution dielectric spectroscopy the guest dynamics of tetrahydropyran, cyclopentane, trimethylene oxide, 1,3-dioxolane, and 1,4-dioxane clathrate hydrates is studied. By investigating lattice-deuterated as well as guest-deuterated crystals, the anisotropic guest reorientation is scrutinized and compared with previous results for tetrahydrofuran clathrate hydrate. The reorientational energy barriers depend linearly on the size of the guest molecules except for the clathrate hydrate of cyclopentane, a molecule which exhibits a dipole moment of the order of 0.01D. The ether oxygens of the other guests can induce Bjerrum L defects on the hydrate lattice. Their concentration is examined for ammonia-doped tetrahydrofuran clathrate hydrate. Covering a wide range of NH3 concentrations x, a minimal overall Bjerrum defect concentration is observed that leads to a maximum mobility on the hydrate lattice at x≈0.03%. To examine guest-induced Bjerrum L defects further, the translational motion on the undoped hydrate lattices is studied using stimulated-echo spectroscopy: The proton dynamics of CP clathrate hydrate is virtually identical to that of hexagonal ice and the water motion of THP clathrate hydrate closely matches that of the tetrahydrofuran compound.