Molecular reorientations of bicyclo [2, 2, 2] octane in its plastic solid phase : correlation times from incoherent quasielastic neutron scattering study

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Abstract
Using the incoherent quasielastic neutron scattering technique, we have studied the molecular reorientations of bicyclo [2, 2, 2] octane CH(CH2CH2)3CH in its plastic phase. A sample temperature ranging from 173 K to 359 K was used with four values of incoming neutron wavelength, using a polycrystalline sample. The observed quasielastic spectra are described by a model in which the molecules perform reorientational jumps between six equilibrium positions equally spaced by 60° around the molecular symmetry axis, together with a tumbling of the molecule from a [111] lattice direction to an equivalent position. The resulting values of the correlation times follow the two Arrhénius laws : τM6 = 1.743 × 10-13 exp(ΔHM/RT) s ΔH M = 5.64 kJ/mole τC4 = 1.952 × 10-13 exp(ΔHC/RT) s ΔH C = 8.90 kJ/mole. For the highest temperatures, a description on the basis of the rotational diffusion model is also examined