A density-functional study of the adsorption of methane-thiol on the (111) surfaces of the Ni-group metals: II. Vibrational spectroscopy

The vibrational eigenstates of methane-thiol (CH3SH) and methane-thiolate (CH3S) in the gas phase and in dense monolayers adsorbed on the (111) surfaces of the Ni-group metals have been investigated within the framework of density-functional theory using generalized response and force-constant techniques. For isolated CH3SH good agreement of eigenfrequencies and intensities with the measured infrared spectra is achieved. For the CH3S radical, experimental information from laser-induced fluorescence spectroscopy is available only for selected eigenmodes. The theoretical predictions show reasonable agreement for the C–H deformation and C–S stretching modes, but predict much higher C–H stretching frequencies in better agreement with estimates based on the vibrational fine structure of the photoemission spectra. For methane-thiol monolayers on Ni(111) and Pt(111) the calculations predict stronger red-shifts of the S–H and C–S stretching modes than reported from high-resolution electron energy loss spectroscopy (HREELS) on condensed multilayers which average over the first layer adsorbed on the metal and further physisorbed molecular layers. For methane-thiolate monolayers the calculations predict modest blue-shifts of the C–H stretching and rocking modes and for the asymmetric C–H deformation modes. Red-shifts are predicted for the symmetric C–H deformation and for the C–S stretching modes. Reasonable agreement with HREELS is achieved. The increased differences between symmetric and asymmetric C–H stretching and deformation modes induced by the adsorption is a consequence of the strongly tilted adsorption geometries.