The energetics of protonation of pyridine and a series of its derivatives, as well as the energetics of formation of hydrogen-bonded N...H...N homocomplexed cations in systems involving substituted pyridines and conjugate cationic acids were investigated by means of restricted Hartree-Fock (RHF) and Møller-Plesset (MP2) ab initio calculations. The Gaussian functional basis set 6-31G* was employed to calculate energy and Gibbs free energy of protonation and cationic homoconjugation in the gas phase. The proton potential of a homocomplexed pyridine cation exhibits a double minimum with a 5.7 kcal/mol energy barrier, which could be reduced to 2.7 kcal/mol by accounting for a thermodynamic correction factor. The calculated protonation energies, ∆E prot , and Gibbs free energies, ∆G prot , have been found to correlate well with the acid dissociation constants (expressed as pK a values) in acetonitrile. On the contrary, the calculated energies, ∆E BHB + , and Gibbs free energies, ∆G BHB + , of formation of the homocomplexes do not correlate with the cationic homoconjugation constant values (expressed as log 10 K BHB + ) determined in acetonitrile.

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