Effects of Au₆ and Au₂₀ Adsorption Sites of Cyromazine–Au Complexes by Raman Spectroscopy and Density Functional Theory

Cyromazine, when used as an insect growth regulator and low-toxicity insecticide, may degrade into melamine and pose a potential threat to the environment and soil health, which has thus attracted extensive research on eliminating such a harmful effect. In this paper, density functional theory (DFT)/LC-BLYP/6-311G(d,p) is used to optimize the geometric structure and analyze the vibration of cyromazine. The DFT/LC-BLYP/def2-SVP is used for the cyromazine–Au complex optimization and vibration analysis. The molecular electrostatic potential (MEP), frontier molecular orbitals (FMOs), vibration frequency, electrophilicity-based charge transfer (ECT) descriptor, binding energy (BE), polarizability, normal Raman spectroscopy (NRS), and surface-enhanced Raman spectroscopy (SERS) of cyromazine adsorbing on Au 6 and Au 20 are calculated. The study of the chemical enhancement mechanism of SERS of cyromazine at different adsorption sites of Au 6 or Au 20 confirms the existence of a charge transfer between cyclopromazine and Au 6 and Au 20, which can adsorb and form stable cyromazine–Au complexes. The results show that N2, H13, and N4 are the adsorption sites of Au 6 and Au 20 . The Raman spectra of the cyromazine–Au complex can be selectively enhanced with a factor up to 9.07. Compared with those of cyromazine–Au 6, the Raman spectra of cyromazine–Au 20 are enhanced more significantly.