Relationship between the Electronic Structure and Acidic-Basic Properties of 4-Substituted Pyridine N-Oxides

CNDO/2 and MNDO calculations on pyridine N-oxide (PyO), 4-nitro- (4 NO 2 PyO), 4-methyl-(4 PicO), 4-methoxy- (4 MeOPyO), and 4-N′,N′ -dimethylamine-pyridine N-oxide (4 N′,N′Me 2 PyO) and on their protonated forms have been carried out. Unknown geometries have been obtained by optimization of the MNDO energy. It has been shown that for 4 NO 2 PyO, 4 MeOPyO, and 4 N′,N′Me 2 PyO N-oxide oxygen protonated forms are much more energetically stable than the forms protonated on substituents. The calculated ionization potentials and dipole moments of the compounds studied are in good agreement with the experimental data. Both CNDO/2 and MNDO protonation energies correlate well with the experimental pK a values determined in four different solvents: acetonitrile, propylene carbonate, nitromethane, and water, the slopes being however much less than unity which indicates the important role of the solvation effect. Based on the parameters of the correlations obtained the protolytic and solvation properties of the solvents studied have been discussed.