Studies of U(C5H5)3Cl: Reorientational motions of cyclopentadienyl rings

Inelastic neutron scattering experiments have been performed on polycrystalline samples of U(C 5 H 5 ) 3 Cl, from 5 K to 300 K, to examine the quasielastic scattering that occurs from the hydrogen atoms in the cyclopentadienyl rings. Analysis of the scattering shows that a reorientational mode consistent with a 2π/5 process is present. At 300 K, the proton correlation time (τ c ) is 4·2 × 10 -12 s and is associated with an activation energy (E A ) of 4·9 kJ mol -1 . Major differences in the dynamic behaviour of the rings have been observed in the following regions: 300 K > T > 245 K; 245 K > T > 78K; 78 K > T.. This is consistent with the observation of structural phase transitions at 78 K and 245 K. Below 245 K, the activation energy, which represents the potential barrier between sites, increases from 4·9 kJ mol -1 to 9 kJ mol -1 . At the same time, 1/3 of the rings are blocked by intermolecular hydrogen bonds that arise during this first transition. At 78 K the second structural phase transition induces a triclinic symmetry (coupled with a twinning), and probably results from a 'freezing' of the cyclopentadienyl rings.