Protonenresonanzspektren von Pteridinen (VI). Mono- und Dikationen von 2-Amino-4-oxo-3, 4-dihydropteridinen

Proton NMR. spectra of 18 pteridines have been measured in strongly acidic solutions. In trifluoroacidic acid mono-cations are formed whereas in fluorosulfonic acid double protonation occurs. For the first time di-cations of pteridines are described, and the structures of these species are determined by NMR. and UV. data. The chemical shifts of the NH₂ protons in the mono-cations and of the methyl protons in the mono- and di-cations follow linear correlations with the basic pK_a values. From NMR. variable temperature measurements the energy barrier to internal rotation in the amidinium system has been ddtermined for both cations. The results are discussed with particular regard to the structure of the mono-and di-cations.