Several C (5)-substituted 2, 4, 6-triaminopyrimidine derivatives and their HBF4 salts were synthesized to study the carbon protonation of the pyrimidine ring. NMR investigations in DMSO-d 6 prove experimentally that, in addition to the usual protonation at N (1), the compounds can be protonated at C (5) as well. We present several new stable cationic σ- complexes in the pyrimidine series, where C (5) protonation predominates over N (1) ...