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

Abstract

Inelastic neutron scattering experiments have been performed on polycrystalline samples of U(C₅H₅)₃Cl, 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.