Guanidinium‐Type resonance stabilization and its biological implications. I. the guanidine and extended‐guanidine series

An unusual type of π‐electron delocalization in Y‐shaped molecules related to guanidine and its protonated form, the guanidinium ion, has been studied by ab initio methods at the STO‐3G and 3‐21G levels. Results are reported for tautomeric, rotameric, and protonated forms of the oxygen‐substituted guanidine series (urea, carbamic, and carbonic acids); “extended‐guanidine” (aminomethylene guanidine) including pseudocyclic forms; and simple ring systems in which the extended‐guanidine group is incorporated (3‐amino‐1,2,4‐triazole, 2,4‐diaminopyrimidine). Both the guanidine and guanidinium type stabilizations have been characterized in terms of a number of structural and energetic parameters: degree of single/double bond character from bond lengths and π‐bond orders, electron distributions, and protonation energies. The major finding is that the structural and energetic properties of the isolated extended‐guanidinium group resemble those of the group when incorporated within 6‐membered heterocyclic or heterobicyclic rings, although the details vary with the nature of the ring and possibility of reinforcement or interference with the substructure resonance from overall ring delocalization. The implications for stabilization of the protonated forms of some biologically important pteridines is discussed.