Tautomerism and Physical Properties of Pyrido[1,2-a]benzimidazole (PBI) GABA-A Receptor Ligands

Structural features of the pyrido[1,2-a]benzimidazole (PBI) chemical series of high-affinity GABA-A receptor ligands were studied by a variety of techniques, including NMR spectroscopy and AM-1 semi-empirical force-field calculations. This analysis revealed that the heterocyclic system exists nearly exclusively in the keto form, with a preference of ca. 8 kcal/mol for the keto over the enol tautomer in the gas phase.