INCOHERENT NEUTRON-SCATTERING STUDY OF THE PSEUDOROTATIONAL AND DIFFUSIVE MOTIONS OF CYCLOPENTANE IN CONDENSED STATE

The quasielastic and inelastic neutron scattering spectra of the solid ordered, plastic and liquid phases of cyclopentane have been recorded in order to analyze the rotational, translational and conformational dynamics of this non-rigid molecule. No diffusive motion with a rate faster than 10(8) s-1 is detected in solid ordered phase III below 122.4K. At 5 K, the twisting mode of the bent molecule is observed at 303 cm-1 and the bending mode tentatively situated at 136 cm-1. In plastic phases II and I, the molecule is shown to undergo reorientations about its pseudo-C5 axis, fluctuations of this axis and the internal motion of pseudorotation. The rate of the former motion is of about 1.2, 2.3 and 3.0 x 10(11) s-1 at 129, 150 and 170 K, respectively. The geometrical and dynamical parameters of the last two motions cannot be determined independently unless the hypothesis is made of a nearly free pseudorotation. The amplitude of the axis fluctuation is then found to vary from 18-degrees at 129 K to 48-degrees at 170 K and its rate from 7.6 x 10(10) to 2.4 x 10(11) s-1 between the same temperatures. In the liquid state at 189 K, the translational diffusion and isotropic rotation of the whole molecule have been characterized. Again it has been necessary to add the pseudorotation contribution to achieve a better fit of the observed spectra. The pseudorotational diffusion constant is found to be of about 8 x 10(11) s-1, as in the plastic phases. This value corresponds to nearly free pseudorotation.