A DFT study of pyrazine derivatives and their Fe complexes in corrosion inhibition process

• The electronic parameters of 3 pyrazines was studied with DFT.
• Electronic parameters showed a good correlation with corrosion inhibition efficiency.
• Four types of interaction of the pyrazine molecules with Fe atoms were considered.
• A large deformation of the ring occurs when Fe interacts in Fe–π interaction.

The DFT/B3LYP calculations were applied to investigate the relationship between electronic properties and corrosion inhibition efficiency of three pyrazine derivatives, 2-methylpyrazine (MP), 2-aminopyrazine (AP) and 2-amino-5-bromopyrazine (ABP). To take into account the solution acidity in experimental conditions, all possible mono-protonated forms, that is protonation at N1, N4 and NH2 sites, as well as the non-protonated form were considered. The molecular orbital analysis showed a good correlation between EHOMO, ELUMO and ΔE (EHOMO − ELUMO) with inhibition efficiency of the three pyrazine derivatives. Four types of interactions between iron and pyrazine molecules, i.e. Fe–π, Fe–N1, Fe–N4 and Fe–NH2 were included in the calculations. As a new approach to this system, inhibition mechanism of the three pyrazine molecules has been discussed in detail based on these four types of interactions. It was found that all four interactions are energetically important. The flat pyrazine ring was substantially deformed followed by a Fe–π interaction. The calculated binding energy of ABP in all forms was found to be higher than two other pyrazines, which is consistent with experimentally observed highest corrosion inhibition efficiency. The lack of Fe–NH2 interaction for MP molecule seems to be the reason for its lower corrosion inhibition efficiency.