Expansion and Shrinkage of Water Cages in Structure II Clathrate Hydrates: The Effects of Temperature and Guest

Camille Y. Jones, Hamilton College, Clinton, NY 13323 In our studies of the structures and properties of hydrates containing additives such as NaF and KF, we have recently exploited two properties of fluorine, the fact that it has only one naturally-occurring isotope, 19F, and the large chemical shift anisotropy of 19F that makes it extremely sensitive to its local environment, including the identity and isotope ratio of the solvent, and concentration of dissolved salts. NMR is an established technique for studying guest dynamics and determination of cage occupancy. Studies have been conducted on proton, deuterium, carbon-13, and xenon-129. To our knowledge, there has never been a hydrate study involving 19F NMR or discussing its binding in the host of a type II hydrate. However, fluoride is known to exist within the water host of a semiclathrate (Fig. 1), and we suspect its hydrogen-bonding ability may allow it to fit into the host of a type II hydrate (Fig. 2).  We conducted a preliminary 19F magic-angle spinning (MAS) solid-state NMR study at the National High Field Magnet Laboratory (NHFML) in collaboration with Prof. M. Cotten, a biophysical chemist at Hamilton and Dr. Riqiang Fu at NHMFL. These results are presented in Fig. 3. In comparing the TBAF hydrate spectra A and B, we note that the tetragonal 32-hydrate has more features than the cubic 28-hydrate, indicating that the 32-hydrate has more inequivalent F- ion environments. Spectra A and B both share resonances in common with the spectrum of the cubic type II THF hydrate made with 0.05 molal NaF in H2O. The spectra of the TBAF 28-hydrate and THF 17-hydrate are the most similar; in particular, (3) the THF 17-hydrate has 3 inequivalent oxygen sites (multiplicity ratio of 96:32:8), matching the number of F- environments indicated in spectrum C. With this in mind, (4) the similarities of spectra B and C may indicate that the TBAF 28-hydrate structure (not published) has a symmetry and geometry similar to that of the sII hydrate. Moreover, (5) except for a very small spinning side band in spectrum D, the spectra indicate that the F- present in the samples exists in relatively symmetric environments. And (6) based on the chemical shifts, F- does not appear to be in direct contact with Na+.  Other work in our lab demonstrates that bulk solution properties may provide information on how hydrates form, but the events taking place at the level of the local structure cannot be further ascertained from solution studies alone. We have compelling evidence from 19F SS-NMR that F- ions incorporate into the host network, where their lack of influence on water structure beyond the first coordination sphere of water may provide a passive probe of hydrate structure, formation, and possibly even memory effect.

Fig. 3. 19F SS-NMR spectra for TBAF 28-hydrate (A) and 32-hydrate (B), and sII THF hydrates with H2O (C) and D2O (D).

Fig. 1. Cations (blue) and anions (green) in tetraisoamylammonium fluoride hydrate.

Fig. 2. Depiction of a fluoride ion as it might exist in a tetrahedrally-coordinates environment within a hydrate.