Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

Spectroscopic studies of the biologically active compound 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), have been performed. Absorption studies in the UV-Vis region for FABT in polar solvents, like water or ethanol, exhibit the domination of the enol form over its keto counterpart, with a broad absorption band centered around 340 nm. In non-polar solvents such as n-heptane or heavier alkanes the 340 nm absorption band disappears and an increase of the band related to the keto form (approximately 270 nm) is observed. Fluorescence spectra (with 270 nm and 340 nm excitation energies used) show a similar dependence: for FABT in 2-propanol a peak at about 400 nm dominates over that at 330 nm while in n-heptane this relation is reversed. The solvent dependent equilibrium between the keto and enol forms is further confirmed by FTIR and Raman spectroscopies. As can be expected, this equilibrium also shows some temperature dependences. We note that the changes between the two tautomeric forms of FABT are not related to the permanent dipole moment of the solvent but rather to its dipole polarizability

Fungicidal evaluation of substituted 4-(1,3,4-thiadiazol-2-yl) benzene-1,3-diols

Fungicidal properties of 4-(1,3,4-thiadiazol-2-yl)benzene-1,3-diols set under in vitro conditions against five phytopathogenic fungi have been evaluated. The substitution panel includes amino-, alkyl-, alkoxyl-, aryl- and heteroarylderivatives. The most active compound with the benzyl substituent exhibits fungistatic effects amounting to 90-100% with the concentration of 20 μg mL-1 against R. solani, similar to the standard fungicides. The derivatives with amine moiety generally display lower activity than other analogues. F. culmorum seems to be the most refractory fungus compared to studied compounds. The influence of substitution of C-5 at the constant fragment at C-2 of 1,3,4-thiadiazole ring on the antifungal effect is discussed. To explain differences in the activity the quantum-chemical calculations were made

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