Unlocking superior corrosion inhibition in HCl: thiol-functionalized purine derivatives outperform amino analogues via synergistic electrochemical and quantum mechanisms

The corrosion inhibition performance of purine derivatives (PR, 6N-PR, 2N6N-PR, 2N6S-PR, and 2S6S-PR) on carbon steel in 1.0 M HCl was systematically investigated through electrochemical and quantum chemical approaches. For the first time, this study elucidates the critical role of thiol substituents over amino groups in enhancing inhibition efficiency, with 2,6-dimercaptopurine (2S6S-PR) achieving an exceptional inhibition rate of 85.24% at 0.001 mol/L. Electrochemical impedance spectroscopy (EIS) and polarization analyses revealed a mixed inhibition mechanism dominated by anodic passivation, while Langmuir adsorption thermodynamics (\(\Delta {G}_{ads}^{\theta }\) = − 38.60 kJ/mol for 2S6S-PR) indicated physico-chemical synergistic adsorption. Quantum chemical calculations demonstrated that thiol groups exhibit higher reactivity than amino groups, with the 2-position substitution significantly enhancing π-electron density and planar adsorption activity. Notably, the energy gap (ΔE = 3.25 eV for 2S6S-PR) inversely correlated with inhibition efficiency, highlighting molecular instability as a key driver for strong metal-surface interactions. A novel quantitative structure–activity relationship (QSAR) model integrating HOMO, LUMO, and dipole moment parameters exhibited remarkable alignment with experimental data (R² > 0.99), providing a predictive framework for designing high-performance corrosion inhibitors. These findings bridge the gap between molecular structure and macroscopic inhibition behavior, offering a transformative strategy for sustainable acid corrosion protection.