Interplay of Inter- and Intramolecular Interactions in Crystal Structures of 1,3,4-Thiadiazole Resorcinol Derivatives

Five new 1,3,4-thiadiazole derivatives have been synthesized, and their crystal structures have been determined by single crystal X-ray diffraction. The influence of substituents on molecular geometry and the three-dimensional arrangement of molecules has been studied by means of single crystal X-ray diffraction, fluorescence, UV–vis spectroscopy, and computational methods. The 1,3,4-thiadiazole derivatives occur in two possible conformations in their crystal lattices: with the ortho-hydroxyl group of the resorcyl ring pointing toward the S or the N atoms from the 1,3,4-thiadiazole ring. In the latter conformation, an intramolecular hydrogen bond is created which is energetically favorable for the isolated molecule as confirmed by theoretical calculations. However, for the molecules in the crystal structures in the former conformation, some intermolecular interactions between the neighboring molecules are strong enough to overrule the intramolecular OH···N hydrogen bond. In the case of one of the 1,3,4-thiadiazole derivatives, a significant disorder was observed, and both conformations were present in one crystal lattice in the ratio 80% to 20% for the two conformers, respectively. On the basis of resonance light scattering results, we explain why crystals of 1,3,4-thiadiazole derivatives can be grown from DMSO and are difficult to be grown from methanol solution

Effect of Solvent Polarizability on the Keto/Enol Equilibrium of Selected Bioactive Molecules from the 1,3,4-Thiadiazole Group with a 2,4-Hydroxyphenyl Function

Three novel 1,3,4-tiadiazole-derived compounds with biological-activity, i.e., 4-(5-(methylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (MDFT), 4-(5-(phenylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (PhATB), and 4-(5-(4-chlorophenylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (4-CIPhATB) were characterized with the use of several spectroscopic methods. Detailed UV–vis studies revealed keto/enol tautomerism of the examined compounds. The absorption spectra recorded in nonpolar solvents exhibited bands that were characteristic of keto tautomers, while in polar solvents the enol form is predominant. A number of spectra revealed the presence of both tautomeric forms in the solution. The keto/enol equilibria observed were both solvent- and temperature-dependent. The keto/enol equilibrium was also observed using FTIR spectroscopy. A detailed analysis of the spectroscopic data leads to a conclusion that the solvent-induced tautomerism of the selected compounds from the 1,3,4-thiadiazole group does not depend on the electric dipole moment of the solvent but more likely on its average electric polarizability. Additionally, a clear effect of the substituent present in the molecule on the tautomeric equilibrium in the selected 1,3,4-thiadiazole analogues was noted