Effect of substituent dependent molecular structure on anti-corrosive behavior of one-pot multicomponent synthesized pyrimido [2,1-B] benzothiazoles: Computer modelling supported experimental studies

The present study is designed to demonstrate the substituent dependent molecular structures on inhibition property of four heterocyclic compounds containing nitrogen, sulfur and oxygen atoms for mild steel corrosion in 1 M HCl. Their inhibition effect was measured using weight loss, open circuit potential (OCP), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and atomic force microscopy (AFM) and computational methods. Outputs of the experimental and computation studies showed that their inhibition efficiencies followed the order: PBT-IV (–OCH₃ + –OH) > PBT-III (–OH) > PBT-II (–CH₃) > PBT-I (–H). Among tested compounds, PBT-IV acted as best corrosion inhibitor and showed highest inhibition efficiency of 94.88% at 42.8 × 10⁻⁵ M concentration. Presence of PBTs in the acidic medium causes increase in the value of R_ct and decrease in the value of current density (i_corr). DFT results showed that the presence of electron releasing substituents (–OCH₃, –OH and –CH₃) enhance the electron density on the frontier molecular orbitals responsible for the metal surface-inhibitors interactions. The molecular structures of the investigated compounds possess the ability of high electron donation (high E_HOMO) as well as acceptation (low E_LUMO). Molecular alignment of inhibitors on the metallic surface was determined MD simulations study which showed that they aligned horizontally (flatly) on the metallic surface that offers the high protection ability. Polarization study showed that mainly they acted as a cathodic type of inhibitors. The results of computational and experimental studies were consistent.