Synthesis of new imine compounds bearing indole and 1,3,4-thiadiazole moieties: Spectral properties, DFT studies, tautomerism and bond analysis

Abstract

This study presents the synthesis of two series of indole derivatives linked with thiadiazole rings, derived from 5-styryl-1,3,4-thiadiazol-2-amine in combination with indole-3-carboxaldehyde and indole-5-carboxaldehyde. The chemical structures of the synthesized compounds were characterized using various spectroscopic techniques, including UV–VIS, FT-IR, 1H-NMR, 13C-NMR, and mass spectrometry. The experimental data obtained from FT-IR and UV–Vis analyses were compared with theoretical results calculated at the B3LYP/6–311++G(2d,2p) level of density functional theory (DFT). The effect of the spatial position of the substituents on the electronic and spectral properties of the compounds was systematically investigated. Additionally, QTAIM and IRI analyses was conducted to elucidate the effects of intramolecular interactions on the σ and π bond characteristics. The impact of substituents on bond strength and bond order was assessed through various methodologies, including intrinsic bond strength index and calculations of Fuzzy, Laplacian, Wiberg, and Mayer bond orders. The findings highlight that the methoxy‑substituted compounds exhibit lower HOMO-LUMO energy gaps compared to the nitro-substituted derivatives, indicating differences in stability and reactivity. Furthermore, the calculated bond orders reveal that a single approach is insufficient for interpreting bond strengths, emphasizing the importance of comprehensive analysis methods. With 1H-NMR, IR and UV–Vis spectroscopic data, it was determined that both indole-indoline and imine-enamine tautomeric forms were formed by proton migration along the N[sbnd]H and CH[dbnd]N groups in the indole, imine and thiadiazole rings. The existence of tautomeric structures was theoretically supported by calculating the transition state and relative enthalpy and Gibbs free energy values.