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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