Theoretical Study on Thermodynamic and Spectroscopic Properties of Electro-Oxidation of <i>p</i>‑Aminothiophenol on Gold Electrode Surfaces

Abstract

The electro-oxidation of <i>p</i>-aminothiophenol (PATP) on gold electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman and infrared (IR) spectral simulations has allowed us to reveal the electro-oxidation mechanism and reaction products of PATP on gold electrodes in acidic, neutral, and basic solutions. PATP can be first oxidized to the radical cation PATP­(NH₂^•+) or the neutral radical PATP­(NH^•) depending on the pH of aqueous solutions, and this is the rate-determining step. The radical cation or neutral radical can then transform to the dimerized products through a radical coupling reaction. In the acidic medium, the radical cation reacts with its resonance hybrid through a N–C4 coupling to form 4′-mercapto-<i>N</i>-phenyl-1,4-quinone diimine (D1), which can further undergo hydrolysis to yield 4′-mercapto-<i>N</i>-phenyl-1,4-quinone monoimine (D2). In the neutral medium, the neutral radical reacts with its resonance hybrid through the N–C2(6) coupling to form 4,4′-dimercapto-<i>N</i>-phenyl-1,2-quinone diimine (D3). In the basic medium, the neutral radical reacts with its resonance structure through the N–N coupling to form 4,4′-di­mercapto­azo­benzene (D4). The adsorbed dimer products exhibit reversible redox properties. The calculated standard electrode potentials of the above four species decrease in the order D3, D1, D2, and D4. Finally, the characteristic bands for the surface Raman and IR spectra of D1 to D4 redox pairs are clearly assigned. This study provides mechanistic insight into the electrochemical reaction properties of PATP on metal electrodes