DFT studies on tautomerism of C5-substituted 1,2,3-triazoles

DFT (B3PW91/6-311þþG**), ab initio (HF/6-311þþG**), and single point CCSD(T)/6-311þþG**//B3PW91/6311þþG** calculations were performed to investigate the stability and tautomerism of the C5-substituted 1,2,3-triazoles. Three different tautomers are possible for the substituted 1,2,3-triazoles: N1–H, N2–H, and N3–H. For all the substituents applied, the most stable is the N2–H tautomer. Out of the two less stable tautomers, N1–H and N3–H, the –F, –CFO, –CH3, – CHO, –Cl, –CN, –CONH2, –NH2, –NO2, and –OH substituents stabilize the N3–H tautomer, whereas only the –BH2, –BF2, and –COOH substituents stabilize the N1–H form. The relative stability of the C5-substituted 1,2,3-triazoles tautomerism is strongly influenced by the possibility for intramolecular interactions (both attractive and repulsive) between substituent and protons located either at N1 or N3 atom. For all the molecules studied, the Gibbs free energy at 0 and 298 K was estimated, too. q 2003 Elsevier Science B.V. All rights reserved.