A computational UV–Vis spectroscopic study of the chemical speciation of 2-mercaptobenzothiazole corrosion inhibitor in aqueous solution

Abstract

In order to understand the chemical speciation of 2-mercaptobenzothiazole (MBT) corrosion inhibitor as a function of pH, the experimental electronic absorption spectra was compared with those calculated at the EOM-CCSD, TD-M06-2X and TD-B3LYP levels of theory for several different species that may result from MBT reactions in aqueous solution. The computational spectroscopic analysis was supported by the energetics of tautomerization, deprotonation, non-covalent association and ion-pair formation reactions calculated at the M06-2X/6-31++G(d,p) level of theory. The thioketonic and deprotonated anionic forms of MBT were found to be the main species depending on the pH, while the formation of the ion-pair was also supported under basic conditions, especially when put in perspective of the release profiles of the inhibitor from nanocontainers published in the literature. The calculated energetic and electronic results were used to unveil the tautomeric, acid–base and ion-pair formation equilibrium, relevant to guide the application of MBT as a corrosion inhibitor, and establishing a foundation for future molecular modeling studies concerning the adsorption of MBT onto metal and metal alloys under different pHs.