Basicity of azoles. Part 2. Theoretical study of the basicity of methylpyrazoles and methylimidazoles

Abstract

We present results obtained, using the INDO method, for the intrinsic basicities of 22 diazoles (pyrazoles and imidazoles carrying only methyl substituents). We found a linear relationship between the lone-pair orbital energies and the protonation energies. The presence of methyl groups α to the basic nitrogen atom lowers the aqueous basicity of diazoles by about 1.8 pK_a units. We predict an inversion of the tautomeric equilibria in going from the solution to the gas phase for 3(5)-methylpyrazoles and 4(5)-methylimidazoles.