FT-IRAS of Adsorbed Alkoxides:  Fluorinated Ethoxides on Cu(111)

Abstract

Fourier transform infrared reflection absorption spectra (FT-IRAS) of a set of fluorinated ethoxides (CF n H 3-n CH 2 OH, n ) 0-3) on the Cu(111) 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 (CF 3 CD 2 O-Cu(111)) exhibits a primary isotope effect. Quantitative measurements of the absorption intensities of CF 3 CH 2 O (ad) and CF 3 CH 2 OH 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(111) surface has the C-C bond tilted 70 ( 2°from the surface normal while in trifluoroethoxide the C-C bond is oriented 50 ( 5°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°for ethoxide and 20 ( 11°for trifluorethoxide. These results indicate that the effect of fluorination of the methyl end group is to tilt it away from the surface.

Negative Hyperconjugation versus Electronegativity: Vibrational Spectra of Free Fluorinated Alkoxide Ions in the Gas Phase

“…DFT calculations reveal some unexpected features of gaseous alkoxide ions. Although the experimental IR absorptions in the CH stretching regions of trifluoroethoxide on a copper surface (2950 cm −1 )10 and of sodium hexafluoroisopropoxide in a KBr pellet (2834 cm −1 )11 match the calculated frequencies for cuprous trifluoroethoxide and sodium hexafluoroisopropoxide in the gas phase, respectively, we find that the CH stretches predicted for free gaseous alkoxides shift substantially to lower frequencies, that is, 2481 and 2392 cm −1 for isolated ethoxide, 2424 cm −1 for isolated isopropoxide, 2476 and 2541 cm −1 for isolated CF 3 CH 2 O − , and 2543 cm −1 for isolated ( CF 3 ) 2 CHO − . The apparent weakening of the CH bond adjacent to negatively charged oxygen is consistent with negative hyperconjugation, as depicted in Scheme , where X=H.…”

Inductive Substituent Effects: Metal Surfaces versus the Gas Phase

“…9,10 Because the dipole moment lies along the C-C bond such that the positive end is toward the reaction center, there is a relationship between θ x and φ such that φ ) 90°+ θ x , or φ ) 120°( 10°. The change in charge at the reaction center, ∆q, is chosen to have a magnitude equal to three-eighths of an elementary charge or 0.6 × 10 -19 C and to be located at the -C atom.…”

Reflection Absorption IR Spectroscopy (RAIRS)

“…For p -polarization, the sign of RAIRS bands is changed by crossing the Brewster angle at 73 ° . In addition to the structure and orientation of molecules, reaction kinetics -for example, of alkoxides on Cu(111) surfaces [19] -may also be investigated.…”