Molecular structure, tautomer's, reactivity and inhibition studies on 6-Methyl-2-thiouracil for mild steel corrosion in aqueous HCl (1.00 M): Experimental and Theoretical Studies

Abstract

• Raman, Infrared and NMR spectra were interpreted based on keto-thione tautomer of 6-Methyl-2-thiouracil molecule. • Polarization and impedance measurements were analyzed for corrosion inhibition efficiency of mild steel in HCl. • FMO and NBO calculations were to correlate the active sites with the inhibition efficiency. • Adsorption annealing simulations locate the best adhering geometry of the inhibitor to the metal surface. Raman (3500-100 cm −1 ) and FT-ATR (4000-360 cm −1 ) spectra of solid 6-Methyl-2-thiouracil (6M2TU) have been recorded in addition to 1 H and 13 C NMR along with Mass fragmentation patterns. Moreover, the hydroxy-thiole, keto-thiole, hydroxy-thione and keto-thione tautomer's stability were theoretically probed using (DFT) B3LYP/6-31G(d,p) calculations in favor of keto-thione in the gaseous phase by ~12-24.5 kcal/mol. Moreover, the monomer and dimer structures were verified by MS/MS spectral measurements owing to the recorded m/z peaks at 141.2 and 281.1, respectively. The inhibition efficiency of 6M2TU towards mild steel iron (MSFe) against 1.0 M HCl has been investigated with molar concentrations of 0.75-7.5 × 10 −3 at 30±2°C using potentiodynamic polarization, impedance measurements and electrochemical frequency modulation. The entitled compound shows a superior inhibition capability against corrosion ranged from 72-94% was obtained. The polarization curves suggest a mixed-type inhibitor for 6M2TU obeying the Langmuir adsorption model while the impedance data ensures an increase in the charge transfer resistance upon its adsorption on the surface of FeMS. Natural bond orbital (NBOs) showed an effective charge transfer from the inhibitor to d -orbitals of the metal in the following order; LP (S) > LP (O) > LP (N 1 ) > π (C=S) > LP (N 3 ) > π (C 5 =C 6 ) > π (C=O). The calculated low Δ E g a p (4.21 eV) predicts that 6M2TU have an efficient inhibition efficiency. Finally, The kinetic parameter F number (5.369673 cm −1 ) and the potential function (V 3 ) was estimated at 454±19 cm −1 (1.30±1.05 kcal/mol) implementing B3LYP/6-31G(d,p) optimized SPs and the calculated torsion frequency at 133 cm −1 . Raman, infrared and NMR spectra were used to study the molecular conformational/tautomeric structures of 6-Methyl-2-thiouracil. Vibrational interpretations were suggested for keto-thione tautomer based on normal coordinate analysis. The corrosion its inhibition effect on iron mild steel in HCl was investigated using potentiodynamic polarization and electrochemical impedance measurements favors a dominant/minor chemical/physical adsorption mechanism.