Study on Surface-Enhanced Raman Scattering Spectra of Malachite Green on Silver Surface Based on Density Functional Theory

Density functional theory (DFT) at the PBE0-D3 hybrid functional and 6-311G + (d,p) basis set levels was employed to optimize the ground state of malachite green (MG). The optimized MG molecule has a nonplanar structure. The electrical properties and densities were calculated for MG by the natural population analysis (NPA) method at the same level. GaussView and Multiwfn software were used to draw a Fukui function electron density isosurface coloring map and an electrostatic potential coloring map of the MG molecular surface. We found that the region near the two N atoms in MG was most susceptible to an electrophilic reaction. Time-dependent density functional theory (TD-DFT) was used to calculate the electronic excitation data of MG and draw the UV-Vis spectrum of MG at the same level. By comparing the experimental UV-Vis spectrum with the calculated UV-Vis spectrum, it was found that the UV-Vis spectrum of MG has three peaks, of which the maximum peak is 616 nm (calculated as 586 nm). The structure of the MG-Ag4 complex was optimized by using DFT. The resonance surface-enhanced Raman scattering (SERS) spectrum excited with 633-nm laser light was calculated and compared with the experimental results, and there was a high similarity in the characteristic peak positions between them. Further analysis showed that MG combined with the silver plane by the adsorption orientation of one of the N,N-dimethylaniline skeleton planes was almost perpendicular to the silver plane in SERS detection. One of the N atoms in N,N-dimethylaniline is combined with Ag and then adsorbed onto the silver substrate to form a special surface-bound compound.