Decoding intermolecular interactions of 4-phenyl-5-(2-thienyl)-2,4-dihydro-3 H-1,2,4-triazole-3-thione: Insights from X-ray diffraction and energy decomposition analysis

Understanding the supramolecular architecture of 1,2,4-triazoles requires looking beyond static geometry to the energetic forces driving their assembly. In this paper, the preparation and the first SC-XRD determination of the molecular structure of 4-phenyl-5-(2-thienyl)-2,4-dihydro-3 H-1,2,4-triazole-3-thione are reported. Single-crystal X-ray diffraction (SC-XRD), in addition to Fourier transform infrared (FTIR), proton/carbon NMR ( 1 H/ 13 C-NMR), and ultraviolet-visible (UV-Vis) spectroscopy, was used to confirm the presence of the thione tautomer. Hirshfeld surface analysis, combined with DFT-SAPT and NCI indices, was employed to investigate the hierarchical nature of the crystal packing. Distinct intermolecular contact modes are observed, including N─H···S and cooperative C─H···S, however the primary factor that dictates the crystal engineering in these solids is the formation of the robust, centrosymmetric \:R₂² (8) supramolecular dimeric synthons, primarily via N─H···S hydrogen bonding. Energy decomposition from SAPT calculations shows that the \:R₂² (8) supramolecular dimeric synthon contributes − 65.3 kJ/mol toward the lattice stability, primarily due to electrostatic and induction contributions. This is significantly different than the contribution of the stacking layers, where the primary contributor to the cohesive energy is dispersion forces at -44.1 kJ/mol. Collectively, these results provide a quantitative basis for understanding the supramolecular assembly of thiophene-substituted 1,2,4-triazoles, effectively bridging the gap between static structural observations and their underlying energetic drivers.