Theoretical design, spectroscopic characterization, and surface adsorption mechanisms of novel isatin-based thiosemicarbazone Schiff bases as potential Cu and Zn corrosion inhibitors

This study presents the design, synthesis, and theoretical evaluation of three novel isatin-based thiosemicarbazone Schiff bases ( Ot-1 , Ot-2 , and Ot-3 ) as potential organic corrosion inhibitors. The targeted compounds were successfully synthesized and structurally characterized using FT-IR, ¹H NMR, and ¹ ³C NMR spectroscopy. To assess their chemical reactivity and corrosion inhibition potential on copper Cu (111) and zinc Zn (0001) surfaces, comprehensive computational studies were conducted. Density Functional Theory (DFT) at the B3LYP level with a mixed basis set approach (6–311 +G(d,p) for C, H, N, O, S and SDD with effective core potentials for Br, I) was utilized to calculate global reactivity descriptors, molecular electrostatic potential (MEP), and Fukui functions, while Monte Carlo simulations evaluated adsorption energies in acidic environments. The findings reveal that incorporating polarizable halogen substituents (Br in Ot-2 , I in Ot-3 ) significantly narrows the HOMO-LUMO energy gap (reaching 3.198 eV for Ot-3 ) and increases electrophilicity (ω = 6.032 eV), thereby enhancing the molecules' charge-transfer capabilities. Compound Ot-3 demonstrated the most negative adsorption energy (−3878.31 kcal/mol on Cu (111)) and highly favorable electron transfer parameters, showing a particular affinity for the Cu surface. Topological analyses (QTAIM, RDG, NCI) further corroborated the presence of strong intramolecular charge transfer and highly stable adsorption configurations. Overall, these computational insights establish a robust theoretical framework identifying these novel compounds as promising candidates to guide future experimental corrosion inhibition testing .