Ab initio study of the energetics of asymmetric hydrogen bridge formation in substituted pyridines

Abstract

The formation of heterocomplexed cations stabilised by asymmetric (N⋯H⋯N) + hydrogen bonding in 10 acid–base systems without proton transfer (PT) formed by substituted pyridines was studied by using ab initio methods at the Restricted Hartree–Fock (RHF) and Møller–Plesset (MP2) levels. For comparison, the energy of cationic heteroconjugation was also determined in selected reverse systems characterised by a PT equilibrium. The calculations were performed for the gas phase by using the 6-31G ∗ basis set. The calculated energy parameters for the heterocomplexed cation formation, i.e. the energies, Δ E BHB 1 + (RHF), and the Gibbs free energies, Δ G BHB 1 + (RHF), calculated at the RHF level, as well as the energies of the formation of the systems studied calculated at the MP2 level, Δ E BHB 1 + (MP2), were subsequently correlated with calculated energetic parameters of protonation, Δ E prot(RHF), Δ G prot(RHF) and Δ E prot(MP2), respectively, as well as with experimental p K a DMSO values of proton acceptors (in both cases at fixed basicity of the proton donor) and logarithms of cationic heteroconjugation constants, log K BHB 1 + , determined in dimethyl sulfoxide. The calculated energy parameters of the cationic heteroconjugation in vacuo have been found to correlate linearly with the calculated protonation energies (at fixed proton donor basicity), with experimental p K a DMSO values of the proton acceptors (at fixed p K a DMSO of the proton donor) and with the cationic heteroconjugation constants ( log K BHB 1 + ).