SERS spectrum of gallic acid obtained from a modified silver colloid

• The SERS spectrum of monomeric gallic acid was obtained.
• Spectral SERS reproducibility is obtained by using a modified silver colloid.
• A normal coordinate analysis supports the Raman bands assignment
• The SERS selection rules allow infer about the molecular orientation of the gallic acid on the Ag surface.

Two different crystals of the gallic acid were microscopically separated from a p.a. commercial product. The Raman spectra analysis allowed distinguishing monomeric and dimeric structures. The vibrational wave numbers were computed using DFT quantum chemical calculations. The data obtained from wave number calculations are used to assign vibrational bands obtained in the Raman spectrum. The dimer, characterized as ellagic acid, involves the carboxyl and hydroxyl moieties. The Raman spectrum in water solution of each species is dominated by the monomeric form. A low negatively charged Ag colloid allowed obtain to the best of our knowledge, the first surface enhanced Raman scattering (SERS) spectrum of the gallic acid. The possible electrophilic attacking sites of the title molecule are identified using MEP surface plot study and the orientation of the analyte on the metal surface is proposed tilted to the surface.

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