J. L. Castro

Vibrational study of the metal-adsorbate interaction of phenylacetic acid and α-phenylglycine on silver surfaces

Raman and SERS spectra of phenylacetic acid and alpha-phenylglycine on silver sols have been recorded at several concentrations and pH values. The alpha-phenylglycine has been also studied in D(2)O. The respective vibrational assignments have been proposed and the analysis of the SERS spectra has made it possible to conclude that phenylacetic acid links to the metal through its carboxylate group only, while alpha-phenylglycine links also through its amino group. In both cases the aromatic ring seems to be almost perpendicular to the metal surface. On the other hand, the contribution of the charge transfer (CT) mechanism to the enhancement of the SERS spectra has been calculated as well and it is found to be very important in both molecules. The band most enhanced by this mechanism is that of vibration 8a, mainly in alpha-phenylglycine.

Evidence of deprotonation of aromatic acids and amides adsorbed on silver colloids by surface-enhanced Raman scattering.

The surface-enhanced raman scattering (SERS) of benzoic acid/benzamide and salicylic acid/salicylamide on silver colloids show important wavenumber shifts with respect to the Raman spectrum of the band assigned to mode 1;ν(ring) when adsorbed on the metal surface (ca. +50 cm(-1)). In the case of the acids, this shift is originated by the deprotonation of the carboxylic group in agreement with the well-known fact that aromatic acids are adsorbed on silver as carboxylates. However, the main conclusion of this work is that a similar behavior is found for the respective amides that do not behave as acids in water solution. The here studied aromatic amides are adsorbed as azanions on silver nanoparticles even at pH 7 and link to the metal through the nitrogen and oxygen atoms of the ionized carboxamide group. This is a very surprising result given that amides are not significantly ionized even at pH 13-14. The deprotonation of these amides is not determined exclusively by the pH, but it is mainly caused by the strong affinity of the anionic species to the metal. Therefore, the SERS must be cautiously used as a universal pH sensor if the adsorption occurs through the ionizable group. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that deprotonated benzamide and salicylamide interact with the metallic surface.

SERS and vibrational spectra of aspartic acid.

Abstract The SERS spectra of aspartic acid have been recorded under different experimental conditions. For the first time, three different types of spectra in function of pH and concentration are reported. The results have been interpreted by correlation with the normal Raman spectra of the solid and the aqueous solutions at different pH values. Finally, a force field calculation has been undertaken, transfering the scale factors of the optimized force constants for related molecules.

Surface-enhanced Raman scattering of hydroxybenzoic acids adsorbed on silver nanoparticles.

Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H(2)O and D(2)O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag(n)) as either oxidobenzoate (A(2-)) or hydroxybenzoate (A(-)), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A(-)) at pH >or=5 and link to the metal through the carboxylate group (Ag(n)--A(-)), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag(+)(n)--A(2-)). It is found that the enhanced bands, and especially the mode 8a;nu(ring), are related to the difference between the equilibrium structures of the adsorbate in its ground (A(-)) and CT-excited (A(2-)) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed.

SERS of acrylic acid and methyl derivatives on silver sols

The surface-enhanced Raman scattering (SERS) spectra on silver colloids of acrylic acid and three methyl derivatives crotonic (3-methylacrylic), methacrylic (2-methylacrylic) and 3,3-dimethylacrylic acid, were recorded and analysed at di†erent concentrations. At high concentrations the respective SERS spectra are explained as being due to the adsorption of the respective anionic forms via the p-ethylenic system or via the carboxylate group, except in the case acrylic acid, for which the presence of the propionate ion was also detected. The SERS of 3,3dimethylacrylate is not a†ected by the concentration of the adsorbate, whereas for the remaining acids the SERS obtained at low concentration show the same spectrum as the saturated monocarboxylic acids in dilute conditions. 1998 John Wiley & Sons, Ltd. (

Surface-enhanced Raman scattering of picolinamide, nicotinamide, and isonicotinamide: unusual carboxamide deprotonation under adsorption on silver nanoparticles.

Surface-Enhanced Raman Scattering (SERS) of picolinamide, nicotinamide, and isonicotinamide has been studied on silver colloids at pH⩾7. The wavenumbers of the SERS bands assigned to 1; νring and ν(C-X) vibrational modes show important blue-shifts (ca. +50cm(-1)) with respect to the Raman spectra, whereas the Amide III bands undergo red-shifts up to -50cm(-1). We demonstrate that these shifts are originated by the deprotonation of the carboxamide groups which link to the metal through the nitrogen and oxygen atoms of the respective azanion groups. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that the pyridinecarboxamides interact with the metallic surface in their deprotonated forms as benzamide does.

Photoinduced charge transfer processes in the surface-enhanced Raman scattering of 2,4,6-trimethylpyridine recorded on silver electrode

Abstract Surface-enhanced Raman scattering spectra of 2,4,6-trimethylpyridine recorded on silver at different electrode potentials have been analyzed on the basis of a charge transfer enhancement mechanism. The main feature of surface-enhanced Raman scattering is the enhancement of the band corresponding to the 8a mode at negative electrode potentials. This behavior can be explained by Franck–Condon factors related to a photoinduced electron transfer process between the metal and the adsorbate. The selective surface-enhanced Raman scattering enhancement is related to the differences between the ab initio equilibrium geometries of the electronic levels involved in the resonant mechanism, i.e., the ground states of the neutral adsorbate and the respective radical anion.

Vibrational spectra of phenylacetate and phenylglycinate ions

The force fields of phenylacetate (PAC) and phenylglycinate (PGLY) ions have been calculated at B3LYP/6-31 þ G* level of theory and improved by using Pulay’s scaling methodology by directly transferring scale factors from related molecules. On this basis, a general assignment of the Raman spectra of these ions has been proposed. q 2003 Elsevier Science B.V. All rights reserved.

SQM Force Field of Glycine: Application to The Analysis of Raman and SERS Spectra

The SER spetrum of glycine has been previously studied by Suh and Moskovits (1). Those authors have discussed the results on the basis of an empirical assignment of the spectrum. Given that aminoacids exhibit strong coupling between normal modes we think that a much more complete assignment of the vibrational spectrum is necessary to interpret SERS results. For this aim force fields have been computed with MNDO hamiltonian and thereafter scaled by using Pulay’s SQM methodology (2). The reliability of the fit has been checked with the experimental Raman and SERS data of the respective deuterated derivatives -ND2 and -ND3.

Enhanced Raman Scattering from Pyrazinoic Acid on Silver and Gold Sols

Abstract We report the SERS of pyrazinoic acid on silver and gold sols. The assignments are consistent with bidentate bridging and unidentate coordinations of carboxylate group, respectively.