Semiempirical molecular‐orbital calculations of the basicities of pyrazole, imidazole, and methyl‐substituted pyrazoles and imidazoles: Inductive model of electron release

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Abstract
This work is concerned with the interpretation of the relative base strengths of pyrazole, imidazole, and some of their methyl derivatives. The LACO‐SCF method of Pariser, Parr, and Pople was used to calculate the π energies of protonation of the bases. We found that the usual treatments of the core attraction integrals failed to order the basicities of pyrazole and imidazole, but this failing could be corrected by using a scaling factor with the core penetration integrals. The electronic effect of the methyl substituents was assumed to be wholly inductive, and the inductive modification of the core integrals αp was found to be highly sensitive to the scaling parameter for the core penetration integrals. A good correlation was obtained between the pKa values and the π protonation energies of seven diazoles, but this correlation could not be extended quantitatively to triazoles.

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