Complete basis set calculations on the tautomerism and protonation of triazoles and tetrazole

Highly accurate methods CBS-4M and CBS-QB3 and standard DFT/B3LYP calculations were employed to study the tautomerism and protonation of triazoles and tetrazole in gas phase. Throughout this analysis, a remarkable agreement between CBS methods and B3LYP/6-311++G(d,p) calculations was observed. To account for solvation effects on tautomer stabilities and protonation pathways, Polarizable Continuun Method (PCM) calculations at the B3LYP/6-311++G(d,p) level were performed considering four different dielectric constants ranging from ε = 2 to ε = 25. Calculated activation free energies for 1,2-hydrogen shifts suggest the existence of tautomeric equilibriums between the protonated forms of 1,2,3-triazole and the neutral forms of tetrazole as an explanation for the protonation of these molecules in gas phase and solution.