Adsorption Conformations, Chemical Enhancements, and Band Broadening Effects in the Surface-Enhanced Raman Spectra of Benzyl Mercaptan and Its Derivatives Adsorbed on Silver: A Density Functional Theory Study

The adsorption of benzyl mercaptan and its derivatives (1,n-benzenedimethane-α,α′-dithiol, n = 2, 3, 4; BDMT) can form a good self-assembly monolayer and has characteristic surface-enhanced Raman spectra (SERS) on the surface of silver colloids and electrodes. We used quantum chemical calculations and molecule-metallic cluster models to calculate their optimized structures and Raman spectra on silver surfaces. Our calculated results can be summarized as three points. First, this kind of sulfur-containing compound prefers to adsorb at bridge and hollow sites, and the theoretically calculated Raman spectra match the experimental spectra well. Second, the characteristic peaks of SERS mainly come from the substituent group −CH2S–, which acts as a direct binding group on the silver surface. The strongest Raman peaks are from the vC–S stretching and wCH2 wagging vibrations. Third, based on the energy relaxation mechanism and experimental SERS, the spectral line width mainly comes from the anharmonic vibrational coupling and the vibration-electron coupling effects. Assuming all the bands with the Lorentzian line shape, the line widths of the vC–S stretching vibration in the substituent group are significantly wider than those of the ring breathing vibration of the benzene ring moiety. Our research elucidates that the deep insight of SERS signals can provide information on adsorption configuration and further reveal the nature of interfacial interaction on metal surfaces.