The azido-tetrazole tautomerism in azoles and its relationships with aromaticity and NMR properties

The properties of 41 neutral molecules (azoles) together with 13 transition states as well as of 16 anions (azolates) with seven transition states belonging to the series of azido/azoles, have been calculated at the B3LYP/6-31G(d) and at the G3B3 levels. Energies, NICS, and bond critical points are used to discuss the ring-chain tautomerism of these compounds in relation with the magnetic aromaticity of the azoles (imidazoles, benzimidazoles, pyrazoles, indazoles, 1,2,4-triazoles, 1,2,3-triazoles and tetrazoles). The removal of the N–H proton has a considerable effect on the azido/tetrazole equilibrium and corresponds to a large stabilization (73 kJ mol−1 in average) of the tetrazoles. Cioslowski’s exothermicity is related to the energy barriers (TSs). A study of the E/Z isomerism of the azides, determination of NICS(1) values and AIM analysis were also carried out.