Vibrational properties of surface intermediates in the oxidation of methanol on copper(100)

Fourier transform reflection absorption infrared spectroscopy, normal mode analysis and ab-initio calculations were used to investigate oxidation of methanol on a Cu(100) single crystal surface. RAIR spectra of methoxy on Cu(100) were interpreted without any ambiguities with the help of seven different methoxy isotopomers. An empirical force field, obtained from normal mode analysis, based on a C_3ν symmetry H₃COCu₃ model was optimized using 15 different experimentally observed vibrational band frequencies from five different C_3ν symmetry methoxy species. The force field so obtained was used to calculate the vibrational frequencies for two other C_s symmetry methoxy isotopomers and also to explain the reason for the low intensity of the symmetric CH₃ deformation band in the RAIR spectra of CH₃O on Cu(100). The normal coordinate analysis also allows the position of the unperturbed vibrations in the CH stretch region to be more accurately estimated and helps to explain the weaker Fermi resonance in the CD stretch region of D₃CO. An ab-initio calculation was also performed in order to compare the force fields obtained from the normal mode analysis. It was then found that the best way to obtain a harmonic force field is to combine the features of both methods in order to match the experimental and calculated frequencies. Methanol was also found to form methoxy even at or below 90 K via a two-step process involving a hydroxyl group. Finally, formate was identified as a possible decomposition product of methoxy on Cu(100), which was confirmed using 18O isotopic substitution.