Chemisorption of 3-mercaptopropionic acid (3MPA) on a colloidal silver surface was investigated using surface-enhanced Raman scattering (SERS). The n(SH) vibration has never been observed in the SERS of 3MPA recorded over a wide range of pH. That behaviour enables us to deduce that the -SH group is deprotonated and links to the metal forming an Ag -S bond but, in contrast, the carboxylic or carboxylate groups do not link to the metal. The carboxylic group is preserved even at pH values at which they should be deprotonated and only under rather basic conditions (pH ≥ 8) do the carboxylic groups dissociate. Consequently, the behaviour of 3MPA adsorbed on colloidal silver is analogous to that observed by Kudelski on a silver electrode. Finally, it is observed in all the SERS spectra recorded at any pH that it is the T-conformer, defined around the C(2) -C(3) bond, which adsorbs preferentially.

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This paper summarizes a series of results where the main role of the charge-transfer (CT) mechanism in the surface-enhanced Raman scattering (SERS) of aromatic molecules is demonstrated. This mechanism consists on a photoinduced CT process between nanostructures of the metallic surface and the adsorbed molecules, giving rise to the transient formation of a radical anion of the adsorbate. A general methodology to detect the CT process in SERS is described. It is demonstrated that the SERS features are determined by the differences between the electronic properties of the neutral and the anionic molecular species. On the basis of these results, we propose to use SERS as a tool to characterize such doublet states.

The vibration nu(SH) has never been observed in the surface-enhanced Raman scattering of mercaptoacetic acid recorded in a wide range of pH. This behavior enables us to deduce that the -SH group is deprotonated and links to the metal forming an Ag-S bond as 1-alkanethiols do. On the contrary, the carboxylic or carboxylate groups do not link to the metal and the carboxylic group is preserved even at pH values under which it should be deprotonated. This fact enables the stabilization of the adsorbed monolayer by removing the electrostatic repulsions between -COO(-) groups and by the formation instead of hydrogen bonds between carboxylic groups. Only under rather basic conditions (pH > 8) does the carboxylic groups dissociate, but the nu(s)(OCO) band is neither enhanced nor shifted toward low frequencies.

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