FT-IRAS of adsorbed alkoxides: Fluorinated ethoxides on Cu(111)

Fourier transform infrared reflection absorption spectra (FT-IRAS) of a set of fluorinated ethoxides (CFnH3-nCH2OH, n = 0-3) on the Cu(lll) surface have been obtained. The decomposition reactions of adsorbed mono-, di-, and trifluoroethoxide to gaseous fluoroaldehydes exhibit different reaction kinetics, as previously reported, and the reaction of trifluoroethoxide-d(2) (CF3CD2O-Cu(111)) exhibits a primary isotope effect. Quantitative measurements of the absorption intensities of CF3CH2O(ad) and CF3CH2OH allowed determination of the orientation of the adsorbed trifluoroethoxide. FT-IRAS results indicate that the orientation of trifluoroethoxide is different than that of ethoxide. Ethoxide on the Cu(lll) surface has the C-C bond tilted 70 +/- 2 degrees from the surface normal while in trifluoroethoxide the C-C bond is oriented 50 +/- 5 degrees from the surface normal. In both species the plane of the molecule defined by the C-C-O atoms is tilted toward the surface. This tilt between the plane of the molecule and the plane of the surface normal is 17 +/- 2 degrees for ethoxide and 20 +/- 11 degrees for trifluorethoxide. These results indicate that the effect of fluorination of the methyl end group is to tilt it away from the surface.