Ab Initio Study of Energetics of Cationic Heteroconjugation in Pyridine N-Oxide and Its Derivatives Systems

The energetics of formation of heterocomplexed cations formed by asymmetric O···H···O hydrogen bonding in systems of pyridine N-oxide and its derivatives was studied by means of restricted Hartree−Fock and Møller−Plesset ab initio calculations. The energy and Gibbs free energies of cationic heteroconjugation were calculated using the 6-31G* basis set in which a d polarization function is accounted for. The calculated energies, , and Gibbs free enthalpies, , of formation of the heterocomplexed cations in vacuo have been found to correlate very good with respective calculated energies and Gibbs free energies of protonation of both the proton acceptors (at a fixed pK a value of the proton donor) and proton donors (at a fixed basicity of proton acceptor) and slightly worse with experimentally determined cationic heteroconjugation constants (expressed as log ) determined on acetonitrile.