Structure and protonation study of the imidazo-{1,2-a]-pyrimidine system in 1H Nuclear Magnetic Resonance

The 1H NMR spectra of imidazo [1,2-a]pyrimidine derivatives have been analysed to study the aromatic character and protonation behaviour of this system. By employing the ring currentn model and calculations based on the coupled Hartree-Fock method it can be deduced that a large -electron delocalisation exists in this heterocyclic system and affects the screening constant of the protons. Charge density schemes obtained by SCF techniques agree with the reactivity behaviour of these molecules.A detailed study of protonation carried out by following the 1H NMR spectra both in trifluoroacetic acid and in aqueous hydrochloric acid shows that the most probable site of protonation is N-1, but leaves open the possibility of a rapid exchange of one proton between N-1 and N-8.