An unusually short intermolecular N - H...N hydrogen bond in crystals of the hemi-hydrochloride salt of 1-exo-acetamidopyrrolizidine

The title compound [systematic name: (1R∗, 8S)-2-acetamidooctahydropyrrolizin-4-ium chloride-N-[(1R, 8S)-hexahydro-1H-pyrrolizin-2-yl)acetamide (1/1)], 2(C₉H₁₆N₂O)·HCl or C₉H₁₇N₂O⁺·Cl^-·C₉H₁₆N₂O, arose as an unexpected product when 1-exo-acetamidopyrrolizidine (AcAP; C₉H₁₆N₂O) was dissolved in CHCl₃. Within the AcAP pyrrolizidine group, the unsubstituted five-membered ring is disordered over two orientations in a 0.897(5):0.103(5) ratio. Two AcAP molecules related by a crystallographic twofold axis link to H⁺ and Cl^- ions lying on the rotation axis, thereby forming N - H...N and N - H...Cl...H - N hydrogen bonds. The first of these has an unusually short N...N separation of 2.616(2)Å: refinement of different models against the present data set could not distinguish between a symmetrical hydrogen bond (H atom lying on the twofold axis and equidistant from the N atoms) or static or dynamic disorder models (i.e. N - H...N + N...H - N). Computational studies suggest that the disorder model is slightly more stable, but the energy difference is very small.