1. These are at 326 cm -1 and 620 cm -1 which are respectively assigned to the ν(C1N1N2) and imidazole ring deformation mode. The phenyl ring breathing mode appears at 996 cm -1 and is largely enhanced. The νs(C3C4C5) mode appears at 1048 cm -1 and undergoes significant enhancement. The optimisation of geometry shows that the PaiH molecule has a planar structure. In the surface adsorbed state the in-plane vibrations are significantly enhanced and on the other hand the out-of- plane vibrations undergo intensity reduction. The parallel adsorption geometry can account for this observation. The SNPs being electron-rich a charge transfer is suggested between the SNPs and the PaiH molecule through the π-system. Single Ag-atom model for the SERS study of 1H-2(Phenylazo) imidazole Washim Hossain a, Dilip K Debnath b, C R Sinha c and Uttam K Sarkar a* a Department of Physics & b Department of Chemistry, Malda College, Malda, WB, India c Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, WB, India. * E-mail ID: chairman_uks@yahoo.comchairman_uks@yahoo.com INTRODUCTION Arylazo heterocycles with N-heteroatoms have azoimine functional group. This functional group has interesting properties including electrochemical properties, stabilisation of low- valent metal ions [1], activation of the C-H function of the pendent aryl group [1] in metal coordination states. Arylazo imidazoles are heterocyclic azo compounds suitable for use as switching molecule because of the imidazole group as well as the azo group. Metal complexes of these molecules act as a molecular switch [2]. (a) (b) Fig.1: Optimized structure of : (a) PaiH molecule (b) PaiH molecule adsorbed Here we report Raman spectra and surface enhanced Raman spectra (SERS) of 1H- 2(Phenylazo) imidazole (PaiH) for the first time. SERS of PaiH adsorbed on colloidal silver nanoparticles (SNP) at different concentration have been studied. The surface properties of the SNP depend strongly on the concentration and type of the interacting ligands in addition to the geometrical parameter dependence [3-5]. The present study aims at better comprehension of the interaction between the SNP and the PaiH molecule. Raman and UV-VIS spectroscopy are used to investigate eleccton delocalization at different concentrations. COMPUTATIONAL DETAILS The Amsterdam Density Functional (ADF) software has been used for theoretical calculations and simulation of the Raman spectra in different conditions. (a) (b) Fig.3: Isosurface : (a) PaiH molecule (b) PaiH molecule adsorbed in Ag-sol The optimized geometry (Fig.1a and b), HOMO-LUMO (Fig.2a and b), isosurface (Fig.3a and b), charge densities on different atoms of the molecule etc. have been explored using the ADF package. ADF package has also been utilized for the assignment of the Raman bands. In the present work, the GGA:BLYP functional has been used for SCF calculations and for post-SCF energy evaluations a triple-ζ polarized slater type (TZP) basis set from the ADF basis set library has been used. The DFT calculations of isolated PaiH molecule and its complex with single Ag atom have been performed. The single Ag-atom model is not an adequate model to explain, quantitatively, about the metal surface coordination. RESULT DISCUSSION The PaiH molecule and its single Ag-atom complex are shown in Fig.1a Fig.1b. Raman spectra of PaiH molecules adsorbed on colloidal silver particles at different concentrations are shown in Fig.4. The Raman signal is significantly enhanced at a concentration of 5x10 -5 M. Monomolecular layer formation is suggested at this concentration and the “first layer effect” [6] is attributed to the large enhancement at this concentration. Fig.5 shows a comparison between the experimentally recorded and theoretically simulated Raman spectra. ADF package has been used for the assignment of the Raman bands exploring the movie frame presentation facility. Some new bands appear at monomolecular layer coverage. REFERENCES [1] Nag J K, Santra P K, Sinha C, Liao F and Lu T Polyhedron 20 (2010) 2253 [2] a) AakerOy C B and Seddon K R Chem Soc Rev 22 (1993)397 b) Steed J W and Atwood J L Supramolecular Chemistry, John Wiley & Sons Ltd, 2000. [3] Welton T Chem Rev 99 (1999) 2071 [4] Mehdi H, Binnemans K, Van Hecke K, Meervelt L V and Nockemann P Chem. Commun 46(2010)234 [5] Wasserscheid P and Keim W Angew. Chem, Int Ed 39 (2000) 3772 [6] Sarkar U K Chem Phys Lett 374 (2003) 341 Fig.5 Raman spectra of PaiH : (a) experimental (b) simulated (a) (b) Fig.2: HOMO : (a) PaiH molecule (b) PaiH molecule adsorbed in Ag-sol Fig.4 SER spectra of PaiH at different concentration of PaiH in silver sols: a) 1x10 -2 M, b) 1x10 -3 M, c) 1x10 -4 M, d) 5x10 -5 M, e) 5x10 -6 M ACKNOWLEDGEMENT The authors gratefully acknowledge the financial assistance of the University Grants Commission, India through the Major Research Project grant no. No. F 41-834/2012(SR).