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 m odified 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 th e adsorbed pyridinethiol and pyrimidine species decreased, and even in an a cid solution the pyridine (or pyrimidine) N atom became less protonated. Th erefore, STM images of self-assembled monolayers of these thiols observed a t properly positive potentials (e.g., > ca. 0.3 V vs. Ag/AgCl for 2-PySH an d 2-PymSH in a 0.1 M HClO4 solution, and >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 str uctures in a neutral solution.