Dynamics of non-rigid molecules. - II. — Quasi-Elastic Neutron Scattering study of liquid cyclopentene

Overview & Synopsis

Quasi-Elastic Neutron Scattering (Q.E.N.S.) measurements have been performed on liquid cyclopentene in order to detect an eventual contribution of the ring-puckering dynamics to the observed profiles. For small momentum transfer values Q and at high resolution, the self-diffusion coefficient has been extracted between 298 and 128 K. Its values are in good agreement with those measured using the NMR spin-echo technique D (cm2. s-1) = 1.3 x 10-3 exp( - 8 653/RT), with R = 8.314 J. degree-1. mole-1. Furthermore, it has been possible to describe reorientations of the whole molecule in terms of spherical rotational diffusion with a Dr value of 6.2 x 1010 s -1 at 180 K. Finally, a precise analysis of the wings of the Q.E.N.S. spectra at higher Q values allows a further component of the motion to be detected. It is due to the internal dynamics and corresponds to a mean jump rate of 1.1 x 1012 s-1 at 180 K. However, the Q . dependence of the width of this component indicates that the ring-puckering process does not occur by instantaneous jumps between two puckered conformations but certainly involves a more complicated diffusive model.