Adsorption of 4-Methyl-4H-1,2,4-triazole-3-thiol molecules on silver nanocolloids:: FT-IR, Raman, and surface-enhanced Raman scattering study aided by density functional theory

Surface enhanced Raman scattering (SERS) in silver nanocolloids and normal Raman spectra (NRS) in the bulk and in aqueous solution of 4-methyl-4H-1,2,4-triazole-3-thiol (4-MTTL) have been investigated. The observed Raman bands along with the corresponding FTIR bands have been assigned from the potential energy distributions (PED) in terms of internal coordinates of the molecule estimated from the output of the DFT calculations. The pH-dependent normal Raman spectra of the molecule in aqueous solution have been recorded to elucidate the protonation effect and preferential existence of the tautomeric form/forms of the molecule in acidic, neutral, and alkaline media. The SERS spectra of the molecule adsorbed on the nanocolloidal silver surface at various pH values are also reported. The appearance of overlapped Ag-N and Ag-S stretching vibrations, considerable red shift of the 1488 cm(-1) band, and enhancement of all the bands principally representing the in-plane vibrations of the A' species of the thione form of the molecule in the SERS spectra suggest that the molecules are adsorbed onto the nanocolloidal silver surface through the lone pair electrons of N-1 and S-6 atoms with the molecular plane tilted with respect to the silver surface at acidic, neutral, and alkaline pH.