Oxidation-State Changes of Molecules Irreversibly Adsorbed on Electrode Surfaces As Monitored by in Situ Fourier Transform Infrared Reflection Absorption Spectroscopy

In situ Fourier transform infrared reflection absorption spectroscopy has been used to monitor the changes in the vibrational properties of 2,5-dihydroxythiophenol (DHT) irreversibly adsorbed on polycrystalline Au as a function of the applied potential. The results obtained have indicated that the normalized difference spectra of DHT in its oxidized state exhibit features consistent with the formation of the quinone derivative of the compound, benzoquinonethiol (BQT), in agreement with the suggestions put forward earlier in the literature on the basis of other measurements. Strong similarities were also found between the normalized difference spectra of adsorbed and solution-phase BQT. The relatively large intensity of the signals observed for the adsorbed species has provided strong evidence that both DHT and BQT are adsorbed on Au with the molecular plane forming a small rather than large angle with respect to the normal to the surface.