Molecular motion in solid bicyclo[2.2.2]octane and bicyclo[2.2.2]oct-2-ene studied by nuclear magnetic resonance

2014 Proton n.m.r. relaxation time measurements have been made on bicyclo[2.2.2]octane (BCO) and bicyclo[2.2.2]oct-2-ene (BCOE). Measurements for each sample were made throughout the plastic and brittle phases, down to 77 K. The results have been interpreted in terms of the various molecular motions occurring in the solid. Below the plastic-brittle phase transition point anisotropic molecular reorientation appears to occur. The activation enthalpy for this motion is 17.1 ± 1.2 and 17.3 ± 1.2 kJ.mol-1 in BCO and BCOE respectively. Above this phase transition a rapid endospherical reorientation is observed and relaxation time measurements yield an activation enthalpy of 7.7 ± 0.9 and 7.4 ± 0.6 kJ. mol-1 for this process in BCO and BCOE respectively. At higher temperatures molecular self-diffusion becomes the dominant controlling mechanism for T103C1 and T2. The activation enthalpy for this process is found to be 95.1 ± 2.1 and 66.8 ± 6.3 kJ.mol-1 in BCO and BCOE respectively. Correlation times, 03C4, for translational and reorientational motion are evaluated. The results and mechanisms for reorientational and translational motion are discussed in relation to the respective crystal structures and thermodynamic properties of the materials. Results related to the molecular reorientation in phase I of BCO are in quantitative agreement with those of incoherent quasi-elastic neutron scattering. The unit cell of the plastic phase I of BCOE which has been investigated by X-ray powder diffraction is f.c.c., with a0 = 0.908 ± 0.002 nm. J. Physique 46 (1985) 271-280 FTVRIER 1985,