Mobility of CD4 molecules in nanoscale cages of zeolites as studied by deuteron NMR relaxation

Deuteron spin-lattice relaxation was applied to study mobility of CD4 molecules trapped in the cages of zeolite NaY. There are two, interconnected sets of cages: α-cages and β-cages with 1.16 and 0.74 nm diameter, respectively. The relaxation temperature dependence, measured between 4 and 300 K, can be divided into four ranges with characteristic motional parameters. At higher temperatures exchange between cages dominates. Increasing rate of translational motion leads to a significant reduction of the relaxation rate. Features typical for quantum rotors were observed at low temperatures. Molecules in the α-cages exhibit reorientational freedom, while motion of these in β-cages is significantly restricted. Increasing abundance of molecules in β-cages indicates slow diffusion down to low temperatures.