A SOLID-STATE C-13 NMR-STUDY OF THE MOLECULAR-MOTION OF ETHYLENE ADSORBED ON A SILVER SURFACE

The reorientation of ethylene on a silver catalyst surface has been studied by solid-state C-13 NMR. The static cross-polarization spectra at different temperatures have been measured. Different jump site models are proposed to simulate the experimental results. It was found that the models involving a low number of jump sites are more sensitive to the experimental details. By comparison of the simulated and experimental results, the 6- and 4-site jump models are chosen as the most satisfactory model to fit the experimental spectra. On the basis of this representation, the activation energy derived for the jump process is 4.3 kJ/mol. From the simulated results, it was concluded that the symmetry axis for the motion of the ethylene at low temperatures (-173 to ca. -45-degrees-C) is perpendicular to the plane of the ethylene molecule. At higher temperatures motion about other axes is initiated such that at room temperature a nearly isotropically averaged C-13 sheilding tensor is observed.