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

Abstract

The vibrational eigenstates of methane-thiol (CH3SH) andmethane-thiolate (CH3S) in the gas phase and in dense monolayers adsorbed on the (111) surfaces of theNi-group metals have been investigated within the framework of density-functionaltheory using generalized response and force-constant techniques. For isolatedCH3SH goodagreement of eigenfrequencies and intensities with the measured infrared spectra is achieved. For theCH3S radical, experimental information from laser-induced fluorescence spectroscopy is availableonly for selected eigenmodes. The theoretical predictions show reasonable agreement forthe C–H deformation and C–S stretching modes, but predict much higher C–Hstretching frequencies in better agreement with estimates based on the vibrationalfine structure of the photoemission spectra. For methane-thiol monolayers onNi(111) and Pt(111) the calculations predict stronger red-shifts of the S–H andC–S stretching modes than reported from high-resolution electron energy lossspectroscopy (HREELS) on condensed multilayers which average over the first layeradsorbed on the metal and further physisorbed molecular layers. For methane-thiolatemonolayers the calculations predict modest blue-shifts of the C–H stretchingand rocking modes and for the asymmetric C–H deformation modes. Red-shiftsare predicted for the symmetric C–H deformation and for the C–S stretchingmodes. Reasonable agreement with HREELS is achieved. The increased differencesbetween symmetric and asymmetric C–H stretching and deformation modes inducedby the adsorption is a consequence of the strongly tilted adsorption geometries.