Theoretical Spectroscopic Study of Normal Raman and Charge Transfer Surface-Enhanced Raman Scattering (SERS) Spectra...

In this work, the discrimination of the enantiomers of cysteine (l- and d-CYS) using the chiral Au₃₄ and Ag₄@Au₃₀ clusters was theoretically investigated in the gas phase and water. Two modes were considered for the interaction of each enantiomer with the clusters (via only its S atom or its S atom and NH₂ group, simultaneously). The interaction energy (E_int) and adsorption energy (E_ad) for the complexation of each enantiomer with the clusters for each interaction mode were calculated. Considering the calculated interaction energies, the interaction of d-CYS with Au₃₄ is stronger than that of l-CYS with the same cluster. Also, it was observed that the substitution of the Au₄ core of the Au₃₄ cluster with the Ag₄ cluster caused the increase of the interaction energy of l-CYS with the Ag₄@Au₃₀ cluster compared to the Au₃₄ cluster, while the reverse trend was observed for d-CYS. Quantum theory of atoms in molecules (QTAIM) analysis was employed to calculate the interaction paths and their related bond critical points (BCPs) between the CYS enantiomers and the clusters to explain the difference between the interaction energy of the enantiomers with the clusters. The IR, normal Raman (NR), and surface-enhanced Raman scattering (SERS) spectra of the enantiomers interacting with the Au₃₄ and Ag₄@Au₃₀ clusters were calculated, and the discrimination between l-CYS and d-CYS using the calculated spectra was explained. It was found that the discrimination of the enantiomers based on their interaction with the clusters is controlled by the charge transfer between the enantiomers and the clusters.