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(C(9)H(16)N(2)O)·HCl or C(9)H(17)N(2)O(+)·Cl(−)·C(9)H(16)N(2)O, arose as an unexpected product when 1-exo-acetamido­pyrrolizidine (AcAP; C(9)H(16)N(2)O) was dissolved in CHCl(3). 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.