Protonization of purine, adenine, and guanine. NMR spectra and structure of mono, di, and tri cations

Proton NMR. spectra of purine, adenine, guanine and methylsubstituted guanines have been measured in CF₃COOH, FSO₃H and FSO₃H? SbF₅? SO₂ at 27° and low temperatures. These conditions permit to study multiple protonation of purines, using chemical shifts of CH, NH and OH protons. The spectra of mono-, di- and tri-cations are described and fully assigned.