QUANTITATIVE ADSORBATE ORIENTATION FROM VIBRATIONAL-SPECTRA - ETHOXIDES ON CU(111)

Fourier-transform infrared reflection absorption spectra have been obt ained for a set of deuterium and isotopic carbon (C-13) labeled ethoxi des adsorbed on the Cu(111) surface at saturation coverages. The frequ ency shifts upon C-13 isotopic replacement indicate that the proper de scription of the skeletal stretches is in terms of CCO modes and not l ocal modes of CO and CC bonds. Comparison of the spectra of the deuter ated ethoxides with gas phase and multilayer spectra of the correspond ing ethanols allowed the determination of the orientation of the adsor bed ethoxide. A solution for the average structure of ethoxide shows t hat the plane of the species is tilted 17+/-2 degrees from the plane o f the surface normal and that the C-C bond, is tilted 70+/-2 degrees f rom the surface normal. The tilt of the C-O bond from the surface norm al is 17+/-2 degrees. At elevated temperatures the ethoxides undergo b eta-hydride elimination and desorb as acetaldehydes. Temperature progr ammed 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.