Ab initio studies of acid-base reactions in the substituted 4-nitropyridine N-oxide systems

The energies and Gibbs free energies of protonation and homocomplexed cation formation were determined by ab initio methods at the RHF, MP2 levels and in the PCM solvation model for 15 4-nitropyridine derivatives using the 6-311+G** basis set. The results of ab initio calculations confirmed that the acidity constants in the non-aqueous media studied changed in terms of their substituent effects and the sequence of acidity changes in water. Furthermore, the values of the cationic homoconjugation energy parameters and equilibrium constants increased with increasing basicities of the N-oxides studied. The proton-transfer energy surface in the homoconjugated cation of the 2-methyl-4-nitropyridine N-oxide exhibits a double minimum, with 2.44 kcal/mol energy barrier. This barrier vanishes when the thermodynamic correction is included.