Exploring the pH dependent SERS spectra of 2-mercaptoimidazole molecule adsorbed on silver nanocolloids in the light of Albrecht's A term and Herzberg-Teller charge transfer contribution

The pH dependent surface-enhanced Raman scattering (SERS) spectra of biologically and industrially significant, 2-mercaptoimidazole (2-MI) molecule at 1.0 x 10(-10) M concentration have been investigated. The pH dependent SERS spectra are compared with the corresponding normal Raman spectra (NRS) of the molecule. The vibrational analyses of the pH dependent NRS spectra of the molecule reveal that in the acidic pH medium, the cationic form of the molecule is preponderant, while in the neutral pH, the existence of the cationic, normal, ylidic, and anionic forms of the molecule are all prevalent. However, in the alkaline pH medium, the anionic form of the molecule is estimated to be dominant. The SERS spectral analyses suggest the predominant adsorption of the cationic and the normal forms of the molecule on the nanocolloidal silver surface at acidic pH of the medium. However, at neutral pH, the cationic and/or normal, ylidic forms of the molecule take active part in the adsorption process, while considerable interactions of the normal, ylidic and/or anionic forms of the molecule with the nanocolloidal silver surface are presaged at alkaline pH of the medium. The genesis of selective enhancements of the Raman bands in the SERS spectra of the molecule recorded at various pH values of the medium has been unveiled from the view of the Albretcht's A and Herzberg-Teller (HT) charge transfer (CT) contribution. (C) 2013 Elsevier Inc. All rights reserved.