Protonation of amino and hydroxypyrimidines. NMR-spectra and structures of mono and dications

Proton-NMR. spectra of amino- and hydroxypyrimidines including biologically important bases have been measured in four solvents: CF₃COOH, CF₃COOH? SO₂, FSO₃H and FSO₃H? SbF₅–SO₂ at 27° and ?55°C. In CF₃COOH mono-cations are formed, whereas in FSO₃H and FSO₃H? SbF₅–SO₂ double protonation occurs. In each case the structures of the protonated species are derived from the chemical shifts of CH, NH and OH protons and proton-proton spin coupling constants. A combination of the measurements described leads to a complete assignment of all proton resonances of the protonated pyrimidines. This approach is also recommended for the structural determination of heterocyclic compounds.