Origin of Substituent Effect on Tautomeric Behavior of 1,2,4-Triazole Derivatives: Combined Spectroscopic and Theoretical Study.

The reaction of 2-aryl-[1,2,4]triazolo[1,5-c]quinazolines with nucleophilic reagents (hydrazine hydrate, sodium hydroxide, sodium methoxide, hydrochloric acid) under acidic conditions leads to formation of compounds that tend to tautomerize. The products of the transformation are distinguished by the position (ortho-, meta-, para-) of the OCH3 group in the aryl moiety. To assign their structures we used the combined approach: experiment and theoretical modeling. The procedure included calculation of the relative stability for possible tautomers, simulation of UV/vis spectra for the most stable forms, and comparison of the resulting curves with the experimental spectral data taking into account the Boltzmann weighting. Through computations, we showed that the orientation of OCH3 substituent remarkably impacts on the tautomeric behavior of triazoles. In the case of ortho-OCH3 it is controlled by formation of the intramolecular hydrogen bond while for meta- and para- derivatives the degree of conjugation plays the decisive role. In order to balance the accuracy and cost of calculations we evaluated the performance of selected DFT methods and 6-31G*, 6-311++G**, and STO##-3Gel basis sets. The last one is a physically justified basis set previously constructed in our group, and its combination with PBE1PBE approach is shown to be the best choice for UV/vis simulations in the frame of the current research.