Anomalous surface‐enhanced Raman scattering of aromatic aldehydes and carboxylic acids

Raman and SERS spectra of benzaldehyde (Bz‐CHO) and chromone‐3‐carboxaldehyde (Ch‐CHO) on silver colloids have been analyzed, being subsequently compared to the spectra of the corresponding acids. In the SERS spectra of both aldehydes, the band corresponding to the ν(CO) stretching mode of the carboxaldehyde group at ca. 1700 cm−1 is missing, while a new band at 1350–1400 cm−1, characteristic of the symmetric stretching mode of carboxylate group, is appearing in both cases. These results point out that aldehydes are oxidized to their corresponding acids when adsorbed on silver nanoparticles. This conclusion has been confirmed by means of HPLC‐MS analysis and supported on the basis of DFT calculations. Copyright © 2016 John Wiley & Sons, Ltd. Benzaldehyde and chromone‐3‐carboxaldehyde are oxidized in silver sols, and their SERS spectra are dominated by the corresponding deprotonated carboxylic acids.