STUDY OF MOLECULAR MOTIONS IN CYCLOHEXANE AND CYCLOPENTANE BY COLD-NEUTRON SCATTERING

The globular compounds cyclohexane and cyclopentane have been studied in the liquid and solid phases by the inelastic scattering of 4 Å neutrons using a rotating-crystal spectrometer. This spectrometer is described and its intensity and resolution are compared with other neutron spectrometers. The broadening of the quasi-elastic peaks as a function of momentum transfer observed at various temperatures has been interpreted in terms of a model proposed by Larsson and Bergstedt, which was modified to include hindered rotational motions of rigid molecules. From a fit of the derived line-width formulas to the measurements, relaxation times for rotational and diffusional motions and average distances covered by a proton during a rotational jump have been obtained. The intensity of the quasi-elastic peaks is mainly determined by the centre-of-mass motions. Debye temperatures for these motions are derived which for cyclohexane are in agreement with values obtained from sound-velocity measurements. The rotational motions of the molecules become more hindered at higher temperatures by the enhanced motions of the surrounding molecules. This behaviour follows also from the observed decrease of the rotational-jump distance with rising temperature. In liquid cyclopentane the rotational relaxation time has a constant value of about 1.3 × 10-12s, which equals the value in the solid near the melting point. The relaxation time for molecular diffusion is defined by an activation energy which turns out to be the same as that for molecular self-diffusion. © 1969.