Potential and pH Dependencies of Adsorbed Species of 2-, 4-Pyridinethiol and 2-Pyrimidinethiol on Au(111) Electrode

Potential and pH dependencies of surface structures of Au(111) electrodes modified with 2- and 4-pyridinethiol (2-PySH and 4-PySH, respectively) and 2-pyrimidinethiol (2-PymSH) were examined by surface-enhanced IR absorption spectroscopy (SEIRAS) and STM. At positive potentials, the pKa values of the adsorbed pyridinethiol and pyrimidine species decreased, even in an acid solution the pyridine (or pyrimidine) N atom became less protonated. Therefore, STM images of self-assembled monolayers of these thiols observed at properly positive potentials (e.g.,> ca. 0.3 V vs. Ag/AgCl for 2-PySH and 2-PymSH in a 0.1 M HClO₄ solution, >0.7 V vs. Ag/ AgCl for 4-PySH in a 0.01 M (M = mol dm^-3) HClO₄ solution) can be used for evaluating the structures in a neutral solution.