Theoretical Studies On Surface-enhanced Raman Spectroscopy (SERS) Of ρ-Aminothiophenol (PATP)

Surface-enhanced Raman spectroscopy (SERS) is one of the most important techniques in studying the nano-structure surface of metals as it has high sensitivity and can obtain high-quality Raman signals. However, the mechanism of SERS is still not clear and is usually explained with the electromagnetic enhancement and the chemical enhancement, the latter one of which is thought to be complicated. The bifunctional structure of PATP plays an important role in understanding the chemical enhancement mechanism of SERS. When PATP adsorbed on roughed silver electrode or silver nanoparticals, the Raman intensities of the b₂ modes are significantly enhanced. Till now, there are three mechanisms to explain this phenomena. But some observations are still not interpreted reasonably. To explore the origin of the b₂-mode enhancement, we studied the Raman spectra of PATP and PATP-metal complexes on the basis of quantum chemical calculations by using Density Functional Theory (DFT). The relevant contents and results are as follows:(1) We calculated the optimized structures of PATP and its different isomers and reassigned all vibrational modes on the basis of potential energy distribution;(2) We analyzed the Raman spectra of PATP-metal complexes. The results show that bonding with metallic clusters has little effect in Raman spectra and there is no strong b₂ modes;(3) We studied the reaction of PATP on metal surface and concluded that the strong bands of so-call b₂ modes is characteristic of 4, 4'-dimercapto-azobenzene (DMAB), which is the dimeric product of PATP.