Theoretical and experimental electronic spectra of neutral, monoprotonated and diprotonated dapsone

Abstract

Dapsone has been receiving growing interest due to it use as a new class of corrosion inhibitors, forming copolymers with aniline, pyrrole, diphenylamine and 4-aminodiphenylamine. The present work focuses on characterizing the theoretical and experimental electronic spectra of the neutral, monoprotonated and diprotonated dapsone. The theoretical electronic spectrum was obtained through the excited state characterization via time-dependent DFT theory. NBO analysis was carried out to identify the molecular orbitals involved in the detected experimental UV bands. The detected absorption wavelength values for the neutral dapsone were 292 and 258 nm (experiment), and 286.9 and 251.7 nm (theoretical). Concerning the protonated dapsone species, it was observed that the absorbance values were strongly dependent on pH, especially at pH between 1.0 and 4.0. As the pH of the solution decreases, the experimental band at 292 nm slowly decreases and shifts to 290 nm, while the absorption at 258 nm decreases until it disappears, and a new absorption band arises at 240 nm. While, when the basicity increases, the absorption spectra of dapsone do not change, but a regular red shift is noticed, and it is continued even up to pH 14.0. The red-shifted spectra may be due to the formation of a monoanion and/or dianion, but the isosbestic points were not constant.