MOLECULAR-DYNAMICS IN LIQUID CYCLOPROPANE .2. RAMAN AND MAGNETIC NUCLEAR-RESONANCE STUDIES OF ORIENTATIONAL MOTION

The orientational dynamics liquid cyclopropane is studied by Raman spectrometry as a function of temperature (155, 300 K) and pressure (up to 3 kilobars). **1**3C and **2H nuclear magnetic resonance experiments are performed in the same temperature range. 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 behavior has been found for other non associated liquids. It results from a decrease of the mean square force acting on the molecular center 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.