Ab initio study of energetics of protonation and hydrogen bonding of pyridine N-oxide and its derivatives

The energetics of protonation and hydrogen-bonded complex formation between the base and its conjugated acid (homoconjugation) of pyridine N-oxide and its selected derivatives was investigated by means of Restricted Hartree-F ock (RHF) and Moller-Plesset (MP2) ab initio calculations. It has been foun d that introducing the d polarization functions (the 6-31G* basis set) is r equired to reproduce correctly the geometry of the N-O bond. The proton-tra nsfer energy surface in the homoconjugated cations exhibits a double minimu m, with 2.4 kcal/mol energy barrier for the pyridine N-oxide homocomplex; t his barrier vanishes when the thermodynamic correction is included. The pro tonation and homoconjugation Gibbs free energies computed in vacuo correlat e well with the acid dissociation and cationic homoconjugation constants de termined in acetonitrile.