LOW-TEMPERATURE CROSS-POLARIZATION MAGIC ANGLE SPINNING C-13 NMR OF SOLID METHANE HYDRATES - STRUCTURE, CAGE OCCUPANCY, AND HYDRATION NUMBER

Spectra of methane trapped in a solid type I hydrate and in a mixed methane/propane type hydrate were studied at minus 80 degree C under cross-polarization and magic angle spinning conditions. Type I could be distinguished from type II structures due to the chemical shift pattern of methane trapped in the large and small cages. In agreement with the statistical theory of the clathrate hydrate formation, methane was found to occupy both large and small cages in each structure to a significant extent. In the case of type I methane hydrate, the distribution of methane over two sites was obtained, and together with a previous determination of the mu potential of the empty lattice with respect to ice, a hydration number of 6. 05 plus or minus 0. 06 was determined.