HIGH-PRESSURE NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE DYNAMIC SOLVENT EFFECTS ON THE ROTATION OF COORDINATED ETHYLENE IN AN ORGANOMETALLIC COMPOUND

The effect of temperature and pressure on the internal rotation rate of coordinated ethylene in π-cyclopentadienylethylenetetrafluoroethylene-rhodium in liquid solution has been investigated by using 1H Fourier transform (FT) nuclear magnetic resonance spectroscopy. The solvents used in this study are n-pentane-d12, carbon disulfide, and methylcyclohexane-d14. The activation energy (13.4±0.2 kcal/mol) for the internal rotation of ethylene is independent of solvent and pressure as determined from conventional Arrhenius type plots and isoviscosity plots. It is found that the rotation of the coordinated ethylene is initially accelerated by pressure, reaches a maximum and then decreases at high pressure. The strong pressure dependence of the observed activation volume for the rotation suggests a strong collisional contribution to the activation volume and the presence of dynamical solvent effects. The experimental data, as interpreted in terms of stochastic models of isomerization reactions, indicate a Kramers' turnover for the pressure dependence of the rotation of coordinated ethylene in the Rh complex in solution. The observation of the energy-controlled regime in this system may be the consequence of the so-called heavy metal atom bottleneck effect which reduces the intramolecular energy transfer within the molecule. The experimental dependences of the rates upon solvent viscosity and/or Enskog collision frequency show that solvent shear viscosity represents only an approximative measure of the coupling of the reaction coordinate to the medium. © 1990 American Institute of Physics.