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 Jyotirmoy Sarkar, † Joydeep Chowdhury,* ,‡ and G. B. Talapatra* ,† Department of Spectroscopy, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India, and Department of Physics, Sammilani MahaVidyalaya, Baghajatin Station, E. M. Bypass, Kolkata 700 075, India ReceiVed: March 19, 2007; In Final Form: May 9, 2007 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. 1. Introduction Vibrational spectroscopy is an important tool for molecular identification. Comparison of Raman and its complementary IR spectroscopy has enabled scientists to elucidate the structural details, protonation effects, and tautomeric preference of complex organic and inorganic molecules over the past decades. 1-5 However, proper assignments of the vibrational wavenumbers and determination of accurate force field for a molecule are of fundamental importance in vibrational spec- troscopy. Recently density functional theory (DFT) is success- fully utilized for the computation of vibrational frequencies and elucidation of structural details of molecules. 6,7 The most intriguing features of DFT are that everything is obtained directly from an observable and we are led to one particle theory that contains electron correlation. 8 The availability of a range of computational tools allows the experimentalist to use computational methods hand-in-hand with experiment to un- derstand the structural and spectroscopic details of molecules. 9,10 Normal Raman scattering, however, is a weak process characterized by cross sections of ∼10 -29 cm 2 . Therefore, normal Raman scattering is often obscured by fluorescence emission. The potential to combine the sensitivity of fluores- cence with the structural information content makes surface- enhanced Raman scattering (SERS) spectroscopy a powerful tool in a variety of fields, including biospectroscopy. 11,12 It has become an increasingly popular technique not only for studying the molecules or ions at trace concentrations down to single- molecule detection level 13-15 but also for estimating the molecular forms and their possible orientations on the metal surface. 16-18 The origin of SERS is broadly explained in terms of electromagnetic 19,20 and chemical interactions. 21,22 Azoles and its derivatives have received much attention in contemporary scientific research because of their remarkable industrial and biological applications. They are a relatively new group of synthetic compounds that have a broad spectrum of antifungal activity. 23,24 Some azoles also have activity against Gram-positive bacteria. Their mode of action is complex and is not fully understood. They are, however, known to inhibit a fungal cytochrome P450 enzyme. 1,2,4-Triazole and its deriva- tives find use in a wide variety of applications, most notably as antifungals such as fluconazole and itraconazole. 25 This mol- ecule also serves as an intermediate in phytosanitary, pharma- ceutical, medicinal, pesticide, photoconductor, and copying systems. Heim et al. 26 in 1955 first observed that intraperitoneal injection of 3-amino-1,2,4-triazole remarkably reduced the catalase activity of the liver and the kidney. Apart from its biological significance, 1,2,4-triazole and its derivative mol- ecules have widespread applications in the field of anticorrosion protection of metals and alloys. 27 Considering the enormous industrial and biological impor- tance, we present here the detailed experimental and theoretical normal Raman spectra (NRS) and SERS and FTIR spectra of the 4-methyl-4H-1,2,4-triazole-3-thiol (4-MTTL) molecule. From a more fundamental point of view, 4-MTTL is also a very interesting compound because of its probable existence in thione-thiol tautomeric equilibrium in the electronic ground state. The pH-dependent normal Raman spectra of the molecule * To whom correspondence should be addressed. For J.C.: telefax, +91- 33-2462-6869; e-mail, joydeep72_c@rediffmail.com. For G.B.T.: phone, +91-33-24734971; fax, +91-33-24732805; e-mail, spgbt@iacs.res.in. † Indian Association for the Cultivation of Science. ‡ Sammilani Mahavidyalaya. 10049 J. Phys. Chem. C 2007, 111, 10049-10061 10.1021/jp072171x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007