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

In order to analyze the influence of substituent groups, both electron-donating and electron-attracting and the number of ?-electrons on the corrosion inhibiting properties of organic molecules, a theoretical quantum chemical study under vacuo and in the presence of water, using the Polarizable Continuum Model (PCM), was carried out for four different molecules, bearing similar chemical framework structure: 2-mercaptoimidazole (2MI), 2-mercaptobenzimidazole (2MBI), 2-mercapto-5-methylbenzimidazole (2M5MBI), and 2-mercapto-5-nitrobenzimidazole (2M5NBI). From an electrochemical study conducted previously in our group, (R. Álvarez-Bustamante, G. Negrón-Silva, M. Abreu-Quijano, H. Herrera-Hernández, M. Romero-Romo, A. Cuán, M. Palomar-Pardavé. Electrochim. Acta, 54, (2009) 539), it was found that the corrosion inhibition efficiency, IE, order followed by the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. Thus 2MI turned out to be the best inhibitor. This fact strongly suggests that, contrary to a hitherto generally suggested notion, an efficient 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 (?E_{Back-donation}) and the inhibition efficiency, IE. The negative values of EHOMO and the estimated value of the Standard Free Gibbs 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, occur spontaneously.