Dynamics of HD Molecules Trapped in Cages of Zeolite

There is considerable interest in the use of mesoporous structures (e.g. zeolites and metal organic frameworks) for hydrogen storage but little is known about the molecular interactions and the dynamics of molecules inside the cages of these structures. In order to determine these properties we have measured the nuclear magnetic spin-lattice and spin-spin relaxation times of HD molecules trapped in the α-cages of zeolite 13X for a wide temperature range 1.6 < T < 20 K. The measurements were made using the technique of nuclear relaxation spectroscopy for which the relaxation process can be described in terms of a set of connected energy baths. The spin-lattice relaxation times depend directly on the heat capacities of the energy baths and thus on the excitations of the trapped HD molecules. Distinct peaks are observed for the spin-lattice relaxation times at low temperatures (T < 6 K) with a marked exponential temperature dependence for T > 12 K, corresponding to the onset of intercage diffusion.