Spectroelectrochemical and DFT approaches to the study of croconic acid adsorption at gold electrodes in acidic solutions

The adsorption of species coming from croconic acid (4,5-dihydroxy-4-cyclopentene-1,2,3-trione, H₂C₅O₅, CA) at gold electrodes was studied spectroelectrochemically in aqueous perchloric acid solutions. Voltammetric experiments suggest the existence of reversible adsorption processes in the potential range between 0.30 and 0.80 V vs RHE. Surface-Enhanced Infrared Reflection Absorption Spectroscopy experiments under Attenuated Total Reflection conditions (ATR-SEIRA) show potential-dependent adsorbate bands in the latter potential range that, according to Density Functional Theory calculations, can be assigned to adsorbed croconate. Calculated optimized geometry corresponds to bonding of croconate species to the gold surfaces involving two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface for low and high crocoanate coverage. For intermediate coverage, lateral repulsions between neighbor croconate adsorbates give rise to the tilting of the molecular plane. The broadening and splitting at high electrode potentials of the C[sbnd]O stretching bands for adsorbed croconate can be explained by invoking the existence of collective vibrational modes appearing at high coverage. The cyclic voltammograms recorded in the CA-containing solutions also show the existence of reversible redox processes at potentials below 0.30 V involving adsorbed species, which are detected in the collected ATR-SEIRA spectra.