Quantitative adsorbate orientation from vibrational spectra: Ethoxides on Cu(111)

Fourier-transform infrared reflection absorption spectra have been obtained for a set of deuterium and isotopic carbon (C-13) labeled ethoxides adsorbed on the Cu(111) surface at saturation coverages. The frequency shifts upon C-13 isotopic replacement indicate that the proper description of the skeletal stretches is in terms of CCO modes and not local modes of CO and CC bonds. Comparison of the spectra of the deuterated ethoxides with gas phase and multilayer spectra of the corresponding ethanols allowed the determination of the orientation of the adsorbed ethoxide. A solution for the average structure of ethoxide shows that the plane of the species is tilted 17+/-2 degrees from the plane of the surface normal and that the C-C bond, is tilted 70+/-2 degrees from the surface normal. The tilt of the C-O bond from the surface normal is 17+/-2 degrees. At elevated temperatures the ethoxides undergo beta-hydride elimination and desorb as acetaldehydes. Temperature programmed reaction spectra for beta-hydride elimination of the selectively deuterated ethoxides reveal both a primary kinetic isotope effect and a measurable secondary isotope effect. (C) 1996 American Institute of Physics.