Hirshfeld, Surface Analysis, Density Functional theory and Corrosion Inhibition Mechanism Proposed of Vanillin 4-ethylthiosemicarbazone on the Mild Steel in 1M HCl

Vanillin 4-ethyl thiosemicarbazone (V4TSC) was synthesised effectively, and single crystal X-ray diffraction was used to validate its structural structure. A monoclinic crystal structure with the P 1 21/n 1 space group is established by the compound. The following are the V4TSC cell's dimensions and volume: V = 1190.3(10) ų, α = 90°, β = 98.083(18)°, γ = 90°, and a = 8.533(4) Å, b = 8.537(5) Å, and c = 16.504(8) Å. Intramolecular hydrogen bond which is N1—H1…N4 and intermolecular hydrogen bonds which are N1—H1…O1, O2—H2…S1, and N3—H3…S1 stabilise the crystal structure. To investigate intermolecular interactions inside the crystal structure, the Hirshfeld surface was analysed. V4TSC has been shown through weight loss, SEM-EDX and AFM studies to be a good corrosion inhibitor in HCl. It forms a protective layer on mild steel surfaces by following the Langmuir adsorption isotherm (R² = 0.999) and exhibiting mixed adsorption (-22.638 kJ/mol). Higher inhibitor concentrations increased inhibitory effectiveness, which occurred at 1 mM with 97.29 %. When mild steel was exposed to the ideal 1.0 mM concentration of V4TSC inhibitors, its surface roughness was considerably lower than it was in the uninhibited solution, as demonstrated by SEM-EDX and AFM images. Spark emission shows that the composition of the S atom on mild steel in V4TSC increased compared to the blank. The optimal structures (thione and thiol) of the compound were computed using density functional theory (DFT) with the 6-311G++(d, p) basis set and the B3LYP technique. DFT was also examined by assessing the chemical reactivity descriptors, frontier molecular orbitals, and molecular electrostatic potential of V4TSC. Comparing the thione and thiol forms of V4TSC, the research showed that the thione form had a greater inhibitory efficiency.