Determination of the geometry of adsorbed unsaturated molecules through the analysis of the CH out-of-plane deformation modes

Abstract

The IR spectra of olefin molecules (ethylene, propylene, 1-butene and isobutene) and of acrylonitrile as an exemple of an olefin with a polar functional group, as well as of aromatic hydrocarbons (benzene and toluene) and of aromatics with polar substituents (benzonitrile and nitrobenzene) adsorbed over transition metal oxide powders (TiO 2, ZrO 2 and Fe 2O 3) have been investigated by Fourier transform IR spectroscopy. For molecules with polar functional groups specific perturbation of the vibrational modes associated with these functions are observed, and show that adsorption primarily involves their interaction with the surface Lewis acid sites. In the case of unsaturated hydrocarbons, the vibrational perturbation is generally weaker, and is interpreted as an indication of the interaction of the molecule through the π electron clouds with the surface. The analysis of the out-of-plane CH deformation vibrations or their harmonics show that they are shifted up strongly for organic molecules adsorbed side-on on the surface. For molecules adsorbed end-on these modes are very poorly perturbed, if at all.