Effect of Aromatic Amine−Metal Interaction on Surface Vibrational Raman Spectroscopy of Adsorbed Molecules Investigated by Density Functional Theory

The adsorption mechanism of aromatic amines on noble metal surfaces and the amine−metal interaction effects on Raman spectroscopy are studied by density functional theory. We find that donation of nitrogen lone-paired electrons plays an important role in the amine−metal interaction. The interaction will be enhanced when aniline adsorbs on positively charged metal clusters because of the electrostatic attraction. It is observed that the Raman signal of the NH2 group is sensitive to the amine−metal interaction. A significant frequency shift and abnormal giant Raman enhancement of the NH2 wagging (ωNH2) mode are predicted theoretically when aniline and its derivatives attach to noble metal surfaces with the amino group. The origin of the frequency shift and the enhanced Raman intensity as well as the line shape broadening has been discussed in detail. Using the characteristic Raman signals of ωNH2, we can infer the adsorption configuration and estimate the strength of the amine−metal interaction. Our calculated results propose that the phenomenon can be extended to other surface probe molecules containing the amino group for investigating amine−metal interactions on SERS active metal substrates.