Theoretical studies on acid-base interactions in the substituted 4-nitropyridines and their N-oxides systems

Acid-base equilibrium constants, involving acidity constants of cationic acids formed by substituted 4-nitropyridines and its N-oxides and cationic homoconjugation constants of the bases studied with conjugate cationic acids (protonated bases) have been studied by ab initio methods. The calculations were carried out in the gaseous phase and after consideration of solvation energy in solutions of a polar protophobic aprotic solvent, acetone. The energies and Gibbs free energies were calculated at the RHF, MP2 and PCM levels and compared to the pKaAC and logKBHB+AC values which were determined experimentally using potentiometrical method. It was found that the calculated energies, ΔEprot, and Gibbs free energies, ΔGprot, of protonation for the systems studied correlate very well with the acid dissociation constants (expressed as pKaAC) of protonated pyridines and their N-oxides. On the other hand the calculated energies, ΔEBHB+, and Gibbs free energies, ΔGBHB+, of homoconjugation do not correlate with the cationic homoconjugation constants values (expressed as logKBHB+AC) determined in acetone.