Quantum Chemical Study of 2-Mercaptoimidazole, 2-Mercaptobenzimidazole, 2-Mercapto-5-Methylbenzimidazole and 2-Mercapto-5-Nitrobenzimidazole as Corrosion Inhibitors for Steel

摘要

In order to analyze the influence of substituent groups, both electron-donating and electron-attractingand the number of electrons on the corrosion inhibiting properties of organic molecules, atheoretical quantum chemical study under vacuo and in the presence of water, using the PolarizableContinuum Model (PCM), was carried out for four different molecules, bearing similar chemicalframework structure: 2-mercaptoimidazole (2MI), 2-mercaptobenzimidazole (2MBI), 2-mercapto-5- methylbenzimidazole (2M5MBI), and 2-mercapto-5-nitrobenzimidazole (2M5NBI). From anelectrochemical study conducted previously in our group, (R. 脕lvarez-Bustamante, G. Negrn-Silva,M. Abreu-Quijano, H. Herrera-Hern谩ndez, M. Romero-Romo, A. Cu谩n, M. Palomar-Pardav.Electrochim. Acta, 54, (2009) 539), it was found that the corrosion inhibition efficiency, IE, orderfollowed by the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. Thus 2MI turned out to bethe best inhibitor. This fact strongly suggests that, contrary to a hitherto generally suggested notion, anefficient corrosion inhibiting molecule neither requires to be a large one, nor possesses an extensive electrons number. In this work, from a theoretical study a correlation was found between EHOMO,hardness (), electron charge transfer (N), electrophilicity (W), back-donation (EBack-donation) and theinhibition efficiency, IE. The negative values of EHOMO and the estimated value of the Standard FreeGibbs energy for all the molecules (based on the calculated equilibrium constant) were negative,indicating that the complete chemical processes in which the inhibitors are involved, occurspontaneously.