Molecular dynamics in liquid cyclopropane. II. — Raman and magnetic nuclear resonance studies of orientational motion

The orientational dynamics of liquid cyclopropane is studied by Raman spectrometry as a function of temperature (155, 300 K) and pressure (up to 3 kilobars). 13C and 2H nuclear magnetic resonance experiments are performed in the same temperature range. The isotropic and anisotropic Raman profiles associated to A'1 vibrational modes allow the determination of the orientational correlation function describing the reorientation of the molecular axis characterized by the diffusion coefficient D〉 . The study of the anisotropic profile corresponding to a mode of E" symmetry with a zero Coriolis constant allows the evaluation of the D∥ constant describing the molecular axis spinning motion. NMR relaxation time T1 of natural abundance 13C under proton decoupling conditions and nuclear Overhauser enhancement allow the determination of the TDD1 relaxation time related to intramolecular dipolar relaxation processes. The validity of the decomposition of the 13C relaxation time is tested by comparison with the T1 relaxation times of 2H ; the quadrupolar coupling constant obtained is in agreement with other NMR measurement in nematic liquid crystal solution. Finally the values and the evolution of TDD1 with the temperature are in good agreement with the D〉 and D∥ deduced from the Raman profiles. This justifies the approximations made in the analysis of the Raman data. This study shows that the orientational motion of the cyclopropane molecules in liquid state is very anisotropic. At low temperature the molecular axis motion seems to be well-described by rotational diffusion. At higher temperatures, the J model seems to be more apt although the mean time between collisions is relatively short. By contrast the spinning motion of the molecular axis is not well described by the diffusion model at all the temperatures. Finally the ratio of the translational and rotational diffusion coefficients is shown to increase with temperature. This behaviour has been found for other non associated liquids. It results from a decrease of the mean square force acting on the molecular centre of mass which is faster than the mean square torque acting on the rotational motion. The latter is probably more influenced by the long range forces.