Translation and reorientation of CD4 molecules in zeolites as studied by deuteron NMR relaxation

Abstract Deuteron spin-lattice relaxation was applied to study translational and rotational mobility of CD 4 molecules trapped in zeolite NaY. Tetrahedral methane molecules are treated as quantum rotators. Relaxation rates related to the intra quadrupole interaction are derived for the T and (A+E) symmetry species in the presence of large tunnelling splittings, consistently with the assumption that A and E species molecules relax at the same rate. An exchange model is presented, which describes the effect on relaxation of CD 4 jumping between two positions characterized by different potentials. There appears also a time-dependent external electric field gradient, which contributes to the deuteron relaxation rate via the electric quadrupole interaction. Spin conversion transitions couple the relaxation of magnetizations of T, A and E spin species. Such a two-step procedure leads to successful fits with the experimental results obtained in the range of temperatures roughly 17–200K for zeolite NaA. Above 200K CD 4 molecules fly freely across zeolite cages and relaxation changes accordingly, while incoherent tunnelling dominates for immobile molecules below 17K. Mobility of CD 4 molecules secluded in zeolites cages provides information on connectivity of cages and allows characterization of surface features.