Computational investigation of a new pyridin-2-yl isomer of thiophene–pyridine 1,3,4-oxadiazole (MTPO) for steel corrosion inhibitor design

This study develops a computational framework for the rational design of corrosion inhibitors for iron surfaces, using the reported inhibitor 2-(5-methylthiophen-2-yl)-5-(pyridin-3-yl)-1,3,4-oxadiazole (MTPO-3) as a reference. Systematic density functional theory (DFT)-based (B3LYP/6-311 ++ G(d,p)) calculations identify its positional isomer, 2-(5-methylthiophen-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (MTPO-2), as the lowest energy isomer, more stable than MTPO-3 by 0.57 kJ · mol −1 . The computational method is validated by reproducing experimental 1 H NMR chemical shifts of MTPO-3 with a small RMSD of 0.30 ppm. Comparative 13 C NMR analysis reveals pronounced chemical shift changes localised on the pyridine ring (P-ring), indicating positional isomer induced electronic redistribution. Time-dependent DFT calculations show that MTPO-2 exhibits a red shifted absorption maximum (λ max = 345.0 nm) relative to MTPO-3 (λ max = 341.71 nm), enhanced oscillator strength, and an additional absorption band at ~ 281 nm absent in MTPO-3. Frontier orbital analysis indicates that while the HOMOs of both isomers are dominated by the thiophene and oxadiazole units, the LUMOs possess substantial P-ring character, imparting partial charge transfer (CT) character to the lowest energy excitations. Ionisation potential (IP) spectra computed using the outer valence Green’s function (OVGF) method further reveal systematically lower binding energies for MTPO-2, with strong spectral correlation to MTPO-3 (RMSD = 0.175 eV), demonstrating that nitrogen repositioning reshapes the occupied electronic manifold through conjugated aromatic coupling. These results show that P-ring positional isomerism modulates both occupied and unoccupied electronic states with clear spectroscopic signatures, providing electronic structure descriptors relevant to inhibitor design. Detailed adsorption studies on iron surfaces are beyond the scope of this work and will be reported separately.