Electrochemical Surface-Enhanced Raman Spectroscopy for Aniline Adsorbed on Silver Electrodes: A DFT Study of the Anharmonic Effects of Amino Wagging Vibrational Modes

Anilinemolecules exhibit a special amino wagging vibrational modeon the surface of the silver electrode. The corresponding Raman peaknot only displays enhanced Raman signals in surface-enhanced Ramanspectroscopy (SERS) experiments but also shows a shift toward higherwavenumbers. This work adopts a molecule-metal cluster model to investigatethe normal Raman spectra of aniline molecules and their SERS spectraadsorbed on silver electrode surfaces by density functional theory(DFT) approaches. Based on the harmonic approximation, the influenceof anharmonic effects on the normal Raman and SERS spectra of anilineis further considered. Our calculated results of the anharmonic Ramanspectra of free aniline molecules show that their anharmonic effectmainly comes from the amino wagging vibrational mode. Then, the anharmonicRaman spectra of aniline binding to neutral and positively chargedsilver clusters were calculated and compared with the harmonic Ramanspectra. It was found that the coupling effect between vibrationalmodes also greatly affects the Raman spectral characteristics. Finally,the excited state energy gap and pre-resonance Raman spectra of theaniline-silver cluster complexes are studied, and it is believedthat there may be a charge-transfer enhancement mechanism.