Surface-Enhanced Raman Spectroscopic Study of Isomeric Formylthiophenes in Silver Colloid

Abstract

Surface enhanced Raman (SER) spectra of 2- and 3-formylthiophenes (2FT and 3FT) in silver colloids are compared with their normal Raman spectra in bulk and in acetonitrile solution. Experimental results indicate adsorption of 2FT on metal surface through the sulfur and the oxygen atoms with the ring plane vertical to the metal surface, whereas the ring plane of the 3FT molecule is inclined to the surface. Moreover, the SER spectrum of 2FT molecule predicts the existence of syn-form of this molecule in the adsorbed state because of large enhancement of the carbonyl stretching mode. Concentration dependence of relative enhancement infers the monolayer formation on metal surface at a specific concentration, implying strong evidence of chemisorption of the adsorbates. The excitation profile of C identical withC symmetric stretching mode of 2FT suggests classical plasmon resonance as the main contributor to surface enhancement. Copyright 1997 Academic Press. Copyright 1997Academic Press

Surface Enhanced Raman Spectroscopic Study of Isomeric Methylthiophenes in Silver Colloid

“…Oligomerization of thiophenes on a nontotally reduced silver surface has been reported by a SERS study (10). In the present study in silver sol, however, we have not observed any oligomerization of halo-and formylthiophenes (11,12) by SERS. All of these thiophenes are adsorbed on the silver surface through the S-atom though in some halothiophenes, the halo atom and in some formylthiophenes, the O-atom are also involved in the adsorption process.…”

“…The Raman excitation profile for many substituted thiophenes, as we reported earlier (11,12), shows resonance with the plasmons in the silver colloid. The contribution of a chemical (or CT) mechanism is also indicated in these methylthiophenes by the appearance of the Ag-S band and by the first layer effect.…”

“…Indeed in our earlier work on chloro-and formylthiophenes (12,23,24) we assigned the 1363 and 1371 cm Ϫ1 bands observed in the bulk liquid to the ring mode involving C 3 -C 4 stretching. In methylthiophenes we have attributed this mode to the 1404 cm (from 1540 cm Ϫ1 in the bulk to 1520 cm Ϫ1 in SERS) in 2-MT compared to only a 10 cm Ϫ1 blue shift in 3-MT on adsorption.…”

Other Molecules with Pharmacological Activity

“…7.9 and Table 7.4 one clearly sees that in the SERS spectrum at pH = 1 the strong band due to the C=O stretching vibration of the anti-form isomer is absent, while in the Raman spectrum this band appears around 1660 cm -1 . In the SERS spectrum of 2-formylthiophene, the band assigned to the C=O stretching vibration is more intense than in the spectrum of 3-formyltiophene (Mukherjee et al 1997), and is determined by the dominance of the cis-form isomer in the surface adsorbed state. In the SERS spectrum of 2-formylthiophene, the band assigned to the C=O stretching vibration is more intense than in the spectrum of 3-formyltiophene (Mukherjee et al 1997), and is determined by the dominance of the cis-form isomer in the surface adsorbed state.…”

“…Therefore, near-infrared excitation was necessary to obtain Raman spectra. The assignment was accomplished mainly by comparison with related molecules (Dollish et al 1973, Katritzky 1963, Mukherjee et al 1997) and using the wavenumbers (unscaled values) and intensities as obtained by the BPW91 method. 7.7 together with the calculated wavenumbers and intensities.…”