Isonicotinic (IN) acid is one of the three monocarboxilic derivatives of pyridine in which the acid group is located in para-position of the heterocyclic ring. It is a weak acid (pK2=4.86) and therefore, it is not completely ionized in neutral aqueous solutions, being the zwitterion and the anion the majority chemical species at neutral pH. In acidic solutions (pK1=1.84) the pyridinic nitrogen atom can be protonated yielding a third chemical species with positive charge [1].
In addition, IN acid shows two functional centres that can interact with the silver metallic surface such as the carboxilate group and the aromatic nitrogen atom. Therefore, the analysis of the SERS spectra of IN has been focused on identifying the chemical species adsorbed on the silver surface and its centre of interaction by considering the participation of a photoinduced charge-transfer (CT) mechanism in each particular SERS record as we have previously detected in the SERS of pyridine derivatives [2].
SERS spectra of the IN acid (5x10-3 M) have been recorded on silver at electrode potentials ranging from 0.00 up to -1.00 V and at different pH by using 0.1 M Na2SO4 aqueous solution as electrolyte. The figure shows the SERS recorded at basic pH. The experimental set up is described elsewhere and the excitation line of 514.5 nm wavelength was used. [2]. The detection of the presence of CT processes, which are similar to resonance Raman, requires to carry out quantum mechanical calculations [2].
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