Investigation of Weak Noncovalent Interactions Directed by the Amino Substituent of Pyrido- and Pyrimido-[1,2-a]benzimidazole-8,9-diones

[Image: see text] Quinones are small redox-active molecules that are able to form intra- and intermolecular interactions both in the solid state and in solution. On the basis of 6-amino-substituted pyrido- and pyrimido-[1,2-a]benzimidazole-8,9-diones, weak interactions were investigated by single-crystal X-ray and (1)H NMR spectroscopy methods. Crystallization of quinone derivatives containing a –NH–CH(2)– fragment led to the formation of both chiral and achiral crystals. The presence of two forms with (endo form) and without (exo form) an intramolecular hydrogen bond was experimentally detected by X-ray crystallography analysis and variable-temperature (VT) (1)H NMR experiments in the cases of isopentylamino- and benzylamino-substituted derivatives. Interestingly, the exo form dominates both in the solid state and in solution.