In search of the weak, six-membered intramolecular hydrogen bond in the solution and solid states of guanidinobenzimidazole

The chemical literature presents evidence for the nonexistence of the intramolecular hydrogen bond in neutral 2-guanidinobenzimidazole, a result that defies chemical intuition. In the current study, analyses of substituted 2-guanidinobenzimidazoles by dynamic ¹H NMR, IR, and X-ray diffraction unveiled the contribution of the intramolecular hydrogen bond to the overall structure and conformational equilibria. The presence of the intramolecular hydrogen bond in this work and its absence in previous studies of the unsubstituted parent compound is reconciled by the fact that intra-molecular hydrogen bonds between the imidazole moieties and guanidino NH₂ protons were weak. The intramolecular hydrogen bonds were more apparent in derivatives with guanidino NHR. The behavior of the latter indicated competition between and coexistence of inter- and intra-molecular hydrogen bonding.