Basicity of azoles. 1. Investigation of total energy of pyrazole and imidazole derivatives by the partitioning method

Conclusions

• 1. Calculations by the total energy partitioning method, applied to a broad group of substituted azoles, have shown that the degree of destabilization of the -π-electron system in heterocyclic compounds upon protonation can provide an explanation for only the difference in principle between the basicities of pyrazoles and imidazoles. Within each series of compounds, the proton affinity is not determined by the magnitude of destabilization of the π-electron system.

• 2. Among the calculated molecular orbital characteristics of pyrazoles and imidazoles — total and π-electron charges on the basic center, their changes upon protonation, the energy of the N-H bond in the cations, the magnitude of change in one-center components of the total energy upon adding a proton, and the charge characteristics of the pyrrole N atom in the ground and ionic states — there is no characteristic that can provide an adequate description of the changes in proton affinity of these compounds.

• 3. For practical purposes in determining the relative basicities of azoles, it is possible to use a correlation of the proton affinity with a readily calculated parameter, the charge on the proton in the cation.