Binding Interaction and Raman Spectra of p−π Conjugated Molecules Containing CH2/NH2 Groups Adsorbed on Silver Surfaces: A DFT Study of Wagging Modes

Abstract

The wagging mode is a characteristic out-of-plane bending vibration for a series of organic compounds containing −CH2/–NH2 groups, such as terminal olefins, p-substituted aniline derivates, and benzyl radicals. The SERS signal of the wagging mode is always sensitive to the interfacial interaction, displaying significant frequency shift and Raman enhancement. To understand the origin of the special SERS signal, density functional theory (DFT) calculation is performed to obtain harmonic vibrational frequencies and Raman intensities of equilibrium structures for the p−π conjugated molecules adsorbed on silver surfaces on the basis of the molecule–metallic cluster model. Our results showed that the frequency shift of the wagging mode is strongly dependent on the hybridization effect, the sp2 changing to sp3 hybridization causes a dramatic frequency shift for the wagging mode. Furthermore, our results also revealed the causes of the remarkable enhancement of this mode in SERS intensity. From the point of view ...