Several medicinally important compounds carry a 2-(thiazol-2-yl)guanidine unit. These species are generally (erroneously) represented as 1-(thiazol-2-yl)guanidine species. Quantum chemical studies were performed to identify the appropriate tautomeric state of this class of compounds. B3LYP/6-31+G(d) calculations indicate the preferred tautomeric state of these species is associated with the 2-(thiazol-2-yl)guanidine structure rather than the 1-(thiazol-2-yl)guanidine structure. G2MP2 calculations on the model system were carried out to study the electronic structure, electron delocalization, and protonation energy; MESP, ELF, HOMA, AIM, and NBO analyses were also carried out. The results indicate that this class of compounds may be treated as species with hidden ::N(←L)R character. Upon protonation of the thiazole ring nitrogen, these systems show the electronic structure as in ::N(←L)2(⊕) systems with divalent N(I) oxidation state.
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