Crystal engineering the clathrate hydrate lattice with NH4F

Abstract

There have been very few attempts to apply crystal engineering approaches to modify the clathrate lattice with the aim of modifying structure and properties. To this end, solutions of ammonium fluoride in water were used to prepare clathrate hydrates with ammonium fluoride replacing water molecules in the hydrate lattice. Both modified structure I Xe and structure II tetrahydrofuran/Xe hydrates were prepared, with the hydrate lattices consisting of NH₄F-water solid solutions containing up to ~19 and 25 mole% NH₄F, respectively. The lattice constants for both hydrates decreased with increase of the amount of NH₄F incorporated, and guest positions and cage occupancies were determined from the PXRD patterns with direct space methods and Rietveld analysis. The ¹²⁹Xe NMR spectra for Xe in the small cavities of each hydrate showed NH₄F concentration-dependent fine structure, not evident in the pure water clathrates, and characteristic of the presence of cage configurations with different distributions of ions. Analysis of the spectra along with density functional theory calculations of the chemical shifts allowed reasonable assignments to be made of the ion distribution. As a test of the altered function of the clathrate upon modification, a clathrate of methanol was prepared, something which has not been possible to do with a pure water clathrate. Structural analysis of the PXRD pattern by direct space methods showed that the methanol OH was hydrogen-bonded to one or both of the NH₄⁺ and F⁻ ions in the lattice, a conclusion corroborated by the absence of molecular motion (except for methyl group rotation) of the methanol guest in the clathrate cages, as determined by static ²H NMR and molecular dynamics simulations. The stability of the modified methanol clathrate can be attributed to the strong methanol CH₃OH⋯F⁻ or CH₃OH⋯NH₄⁺ hydrogen bonding which leaves the water–water hydrogen bonding network intact, as opposed to the situation in a pure water clathrate where the methanol–water hydrogen bonding disrupts the lattice and a stable clathrate cannot be made.