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.