Concentration-dependent orientational changes of 2-amino-2-thiazoline molecule adsorbed on silver nanocolloidal surface investigated by SERS and DFT

The concentration-dependent surface-enhanced Raman scattering (SERS) study of the biologically important, 2-amino-2-thiazoline (2ATH) molecule adsorbed on silver nanocolloids has been investigated. The SERS spectra of the molecule at different adsorbate concentrations are compared with its FTIR spectrum and normal Raman spectra (NRS) in varied environments. The optimized structural parameters and computed vibrational wavenumbers of the tautomeric amino and the isomeric imino forms of the molecule in the gas phase and in methanol solvent have been estimated from density functional (DFT) calculations. The observed Raman signals along with the corresponding FTIR bands have been assigned for the first time from the potential energy distributions (PED) in terms of internal coordinates of the molecule estimated from the output of the DFT calculations. Vibrational analyses reveal that imino species are prevalent in the solid state while the amino species of the molecule are prevalent in methanol solution. The adsorbate concentration-dependent SERS spectra of the molecule adsorbed on a nanocolloidal silver surface are also reported. Considering the entire concentration-dependent SERS spectral profile and the surface, selection rule, it has been predicted that at higher adsorbate concentrations the amino and the Z-isomeric imino form of the molecule attain face-on adsorption geometry. Alternatively, at dilute concentrations, the amino form of the molecule has edge-on adsorption geometry with the molecular plane lying nearly orthogonal to the nanocolloidal silver surface.