The dynamics of molecular reorientation in the disordered phase of bicyclo(2.2.2)octane

X-ray powder diffraction measurements have been made on both ordered and disordered phases of bicyclo-octane. It is concluded that the structure of the ordered phase is hexagonal, in agreement with earlier work. The high-temperature disordered phase is known to be face-centred cubic. Incoherent neutron quasi-elastic scattering measurements have been made throughout the disordered phase. These have been successfully analysed to show that the dominant motions are uniaxial rotations about the unique molecular axes (adequately described as C₆ rotations) plus C₄ rotations by which the unique molecular axes can reorient along the eight different diagonals (111) of the cubic cell. The correlation times are described by: tau ₆ ^-1/s^-1=(1.1+or-0.3)*10¹² exp((-5.3-or+0.5)kJ mol^-1/RT) tau ₄^-1/s ^-1=(3.7+or-1.5)*10¹¹ exp((7.6-or+1.5)kJ mol ^-1/RT).