Neutron-scattering investigation of molecular reorientations in solid cubane

We have studied the reorientations of the cube-shaped molecule cubane (C₈H₈) in its solid phase using quasielastic neutron-scattering techniques both below and above the molecular orientational phase-transition temperature Tₚ=394 K. In the orientationally ordered phase, just below Tₚ, the elastic incoherent structure factor (EISF) extracted from neutron time-of-flight measurements indicates that cubane undergoes π jumps about the molecular fourfold axis. In the disordered phase we find that the EISF excludes isotropic and free uniaxial rotational motions of the molecule. Instead our data suggest cubane undergoes uncorrelated π jumps about the fourfold axis (as in the ordered phase) and 2π/3 jumps about the threefold axis.