Motion of CF3 Groups and Phase Transition in [Cu4(CF3COO)4]·2C6H6 as Studied by 1H and 19F NMR Techniques

The temperature variation of 1H and 19F nuclear spin-lattice relaxation times T1 in [Cu4(CF3COO)4]·2C6H6 was observed at different resonance frequencies. From the NMR experiments, it was found that this complex undergoes a first order phase transition at 130 K, which was confirmed by DTA measurements. In the room-temperature phase, 1H gave normal exponential magnetization recovery curves and logT1 linearly decreased with increasing 103T−1 whereas 19F gave nonexponential magnetization recovery curves, from which long- and short-T1 components could be derived. This indicates that the C3 reorientation of CF3 groups about their respective symmetry axis is responsible for 19F T1 and for 1H T1 as well through 1H-19F cross relaxation. This was confirmed from the coincidence of the logT1 vs. 103T−1 curve of the long component and that of 1H T1. A very small activation energy of 5.5 kJ mol−1 was obtained for the CF3 reorientation. For the low-temperature phase, both 1H and 19F yielded nonexponential magnetization ...