Prediction of preferred protonation sites in pyrrole and its methyl derivatives using molecular electrostatic potentials derived from the PM3 and AM1 methods

The PM3 and AM1 molecular orbital programs have been employed for the investigation of the preferred protonation sites in pyrrole and its N-methyl, 2,5-dimethyl, 3-methyl, 2-methyl and 1,2-dimethyl derivatives. The following are reported: the LCAO coefficients of the HOMO (2p_z) (the HOMO coefficients); the atomic centered charges, which are calculated by the subroutine 'ESP' (ESP charges); the atomic net charges, which are obtained by a Mulliken population analysis (Mulliken net atomic charges). The predictions of the preferred protonation sites made by the HOMO coefficients and ESP charges are in perfect agreement with experimental data, whereas those made by the HOMO coefficients and Mulliken net atomic charges are not in agreement. It is hence obvious that the protonation of pyrrole and its methyl derivatives is controlled by not only HOMO-LUMO interaction but also by electrostatic forces, which are estimated by ESP charges. In other words, the subroutine 'ESP' is useful for estimation of a kind of electrostatic reaction of pyrrole and its methyl derivatives, which suggests that the subroutine may be effective for the investigation of the electrostatic reactions of other compounds.