Dynamics of non-rigid molecules. - I. — 13C T 1 NMR study of the reorientational and ring-puckering motions of cyclopentene in the liquid state

Abstract

The 13 C NMR spin-lattice relaxation time T 1 and Nuclear Overhauser Effect (NOE) of the three distinct carbons of cyclopentene have been measured between 220 and 300 K in order to obtain some information about the ring-puckering motion in the liquid state. The data are first analysed under the hypothesis that the rotational diffusion of cyclopentene resembles that of a rigid asymmetric top. The slower rotation is then calculated to occur around the pseudo-C 5 axis and large differences are found between the three rotational diffusion constants D xx , D yy , D zz . These results seem to have little physical meaning. Furthermore they do not agree with those obtained by infrared, Raman and quasielastic neutron scattering. Therefore the ring-puckering motion has to be considered as an efficient 13 C relaxation mechanism. Although parameters needed to describe both external and internal motions cannot all be determined from the information available one can qualitatively show that at 300 K the molecule executes about (5±4), 10 12 jumps s −1 between its two equivalent bent conformations characterized by a puckering angle of 25 to 30°. The activation energy associated with this process seems to be low since the jump rate is still about (1±0.5).10 12 s −1 at 220 K. On the other hand, the whole molecule rotation is found to be nearly isotropic in agreement with the conclusions of the optical and neutron spectroscopies D r =(0.22±0.02)•10 12 s −1 at 300 K with an activation energy of 5.1 kJ.mole −1 Mesure du temps de relaxation spin reseau et de l'effet nucleaire Overhauser afin d'etudier la dynamique d'inversion du cyclopentene en phase liquide. Le mouvement d'inversion du cycle est considere comme un mecanisme efficace de relaxation de 13 C. La molecule execute environ (S±4).10 12 sauts par seconde a 300 K entre ses deux conformations equivalentes, pliees selon un angle de 25 a 30°