Experimental and theoretical studies of acid-base equilibria of substituted 4-nitropyridine N-oxides

By using the potentiometric titration method, acidity constants in the polar aprotic solvent acetonitrile (in the form of pK(a)(AN) values) of cations obtained by protonation of 13 substituted 4-nitropyridine N-oxides and cationic homoconjugation constants (K-BHB(+)) Of the cationic acids conjugated with the N-oxides studied have been determined. A correlation has been established between the tendency toward cationic homoconjugation (expressed as log K-BHB(+)) and the basicity of the N-oxides in acetonitrile (pK(a)(AN)). Further, by using ab initio methods at the RHF and MP2 levels utilizing the Gaussian 6-31G* basis set, energies and Gibbs free energies have been determined of protonation and formation of homocomplexed cations stabilized by O...H...O bridges in the gas phase. The calculated protonation energies, DeltaE(prot), and Gibbs free enthalpies, DeltaG(prot), in vacuo have been found to correlate well with the acid dissociation constants (expressed as pK(a)(AN) values) of protonated N-oxides, whereas the calculated energies, DeltaE(BHB)(+), and Gibbs free energies, DeltaG(BHB)(+), of homoconjugation do not correlate with the cationic homoconjugation constant values determined in acetonitrile.