Structure and protonation study of the imidazo [1,2-a]-pyrimidine system in1H 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 π-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.