MOLECULAR-MOTION IN SOLID [(CH3)(2)CHNH3](2)BIBR5 AND [(CH3)(2)CHNH3](2)SBBR5 AS STUDIED BY PROTON NUCLEAR-MAGNETIC-RESONANCE

Proton magnetic resonance spin-lattice relaxation time and second mome nt measurements were carried out on polycrystalline isopropylammonium bromobismuthate [(CH3)(2)CHNH3](2)BiBr5 (IPBB) and bromoantimonate [(C H3)(2)CHNH3](2)SbBr5 (IPBA) salts over a wide temperature range. The p roton spin-lattice relaxation measurements in both compounds reveal an asymmetric wide minimum due to the motions of the ammonium and methyl groups. Analysis of the relaxation data yields the activation energy barriers for the motion of the NH3 and CH3 groups. NMR data confirm th e phase transitions, at 155 and 133 K for IPBB, and at 171 and 180 K f or IPBA, known from previous studies. The phase transitions at 155 K i n IPBB and at 180 K in IPBA are connected with freezing of the reorien tation of isopropyl cations. The second moment measurements show that the structure is not rigid on the NMR scale at the lowest temperature studied, 10 K, and that CH3 and NH3 groups execute rotations about the C-3 axis.