In Situ Fourier Transform Infrared Spectroscopy of Molecular Adsorbates at Electrode-Electrolyte Interfaces: A Comparison between Internal and External Reflection Modes

The vibrational properties of 2, 5-dihydroxyben2yl mercaptan (DHBM) irreversibly adsorbed on gold electrodes have been examined in situ in aqueous 0.1 M HClO₄ by two different Fourier transform infrared (FT-IR) spectroscopic techniques: (i) potential difference attenuated total reflection FT-IR (PD-ATR-FT-IR), using a thin layer of gold (~4 nm) sputtered on a (thick layer) Au-pattemed ZnSe internal reflection element as the electrode and (ii) PDFT-IR (external) reflection absorption spectroscopy (PDFT-IRRAS) on a solid Au electrode. The results obtained with both techniques, using the spectrum of the monolayer in the reduced state as a reference, were found to be nearly identical, displaying a set of negative- and positive-pointing features. The first set matched, within experimental error, the most prominent peaks observed in the ATR-FT-IR spectra of the DHBM monolayer using either pure water or 0.1 M HClO₄ as a reference. Furthermore, the positive-pointing features in the PD-FTIR spectra, which correspond to the product of the electrochemical oxidation of irreversibly adsorbed DHBM, were consistent with the presence of a quinone-type moiety, as would be expected on the basis of chemical considerations. These observations indicate that, to the level of sensitivity of these two methodologies, the mode of adsorption and reactivity of DHBM (and probably a number of other species as well) is not appreciably affected by possible differences in the metal surface microstructure.