NMR-STUDY OF THE DYNAMICS OF HYDROGEN IN THE CLUSTER COMPOUNDS TH6HXBR15 (X=5, 7)

The complex dynamical behavior of the disordered hydrogen atoms in the recently discovered cluster compounds Th6H7Br15 and Th6H5Br15 has bee n investigated by means of H-1 solid state NMR, Two different modes of motion can be identified, At temperatures above 250 K the spin-lattic e relaxation is dominated by a thermally activated intercluster jump p rocess with an activation energy of about 4200 K (360 meV), The emerge nce of this process results in an abrupt line narrowing of the NMR spe ctrum at 270 K. Down to the lowest measured temperature of 80 K a loca l intracluster mode is effective. The low-temperature proton spin-latt ice relaxation time is analyzed on the basis of a first principle stoc hastic model for Th6H7Br15. A dynamically disordered and phonon assist ed tunneling process is proposed for the low-temperature dynamics.