FTIR, Raman, and surface-enhanced Raman scattering (SERS) of methimazole (MMI) have been investigated. MMI is an important antithyroid drug that inhibits the formation of thyroid hormone. It is widely used clinically in the treatment of hyperthyroidism, and thus it is useful to study its surface adsorption characteristics. The experimental FTIR and Raman data are supported with DFT calculations using B3LYP functional with LANL2DZ basis set. This is the first report on the vibrational analysis of the thiol and thione forms of MMI and their various possible silver complexes. pH-dependent normal Raman spectra have been recorded, which show the abundance of the thione form of MMI in acidic, neutral, and alkaline media. From the SERS spectra as well as theoretical calculations, it has been inferred that in neutral and alkaline media, the thiol form of MMI is chemisorbed to the silver surface through the ring N atom of the imidazole ring with an edge-on orientation and the imidazole ring lying in the plane of the silver surface. In contrast, it has been concluded that in the acidic medium, the thione form of MMI gets adsorbed to the silver surface. Thus, the pH-dependent SERS spectra have shown the preferential existence of thione and thiol tautomeric forms on the silver surface in acidic, neutral, and alkaline media.

The surface-enhanced Raman scattering (SERS) studies of 5-amino tetrazole (5AT), a tetrazole derivative, in aqueous silver sol at pH approximately 9 and on deposited colloidal silver films were carried out and compared with the normal Raman spectrum of the molecule. The experimentally observed Raman bands along with their corresponding infrared bands were assigned based on the results of density functional theory (DFT) calculations. The significant changes evidenced between the SERS and the normal Raman spectra combined with the theoretical data obtained for Ag-5AT system demonstrated that the molecule is adsorbed on colloidal Ag particles through the lone pair of electrons of the nitrogen atom. The contribution of the chemical mechanism for the SERS enhancement was proved by the behavior of the electronic absorption spectrum of the Ag colloid upon addition of 5AT. This is further supported by the theoretical calculations that show that the favorable interaction of the frontier orbitals localized on Ag(+) and the negatively charged nitrogen from the tetrazole ring leads to the formation of the stable (up to 130 kJ mol(-1)) charge-transfer complex. The orientation of the adsorbed species with respect to the metal surface was also predicted by applying the "surface selection rule". In addition, the feasibility of the formation of the polymeric species has also been discussed.

The SERS spectrum of taurine suggests the predominance of the gauche conformer with sulphonate and amino groups involved in binding.

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