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

4-Hydroxy-1-naphthaldehydes: proton transfer or deprotonation

A series of naphthaldehydes, including a Mannich base, have been investigated by UV-Vis spectroscopy, NMR and theoretical methods to explore their potential tautomerism. In the case of 4-hydroxy-1-naphthaldehyde concentration dependent deprotonation has been detected in methanol and acetonitrile. For 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde (a Mannich base) an intramolecular proton transfer involving the OH group and the piperidine nitrogen occurs. In acetonitrile the equilibrium is predominantly at the OH-form, whereas in methanol the proton transferred tautomer is the preferred form. In chloroform and toluene, the OH form is completely dominant. Both 4-hydroxy-1-naphthaldehyde and 4-methoxy-1-naphthaldehyde (fixed enol form) show dimerization in the investigated solvents and the crystallographic data, obtained for the latter, confirm the existence of a cyclic dimer