Tautomerism of a thiabendazole fungicide on Ag and Au nanoparticles investigated by Raman spectroscopy and density functional theory calculations

The energetic stabilities of a fungicide, thiobendazole (TBZ) on the Au and Ag atoms were analyzed by density functional theory (DFT) calculations at the levels of B3LYP/LANL2DZ. The most stable configuration on Au occurred when the metal atom was close to the N1 atom of TBZ in the same plane. There are two conformers of TBZ, N1-deprotonated and N3-deprotonated tautomers, depending on the hydrogen position of the benzimidazole in TBZ. Due to the planar geometry of the N1-deprotonated tautomer, it is more stable (ΔG = 6.31 kcal/mol) than the N3-deprotonated tautomer, which exhibits N1–C2–C10–C14 and N3–C2–C10–N11 dihedral angles of 30° and 28°, respectively. On the other hand, the most stable geometry of TBZ on Ag was formed when the N3 and N11 atoms of TBZ make a bridged configuration with the Ag atom. The binding energy of TBZ on Au was found larger than that of Ag. Spectral features of TBZ are compared on silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) by means of Raman spectroscopy. The presence of an Ag–N and an Au–N stretching band at 227 and 239 cm−1 in the Raman spectrum indicated that the nitrogen atoms of TBZ may be involved in the binding of the adsorbate on the surfaces of the AgNPs and AuNPs.