An unusually short inter-molecular N-H⋯N hydrogen bond in crystals of the hemi-hydro-chloride salt of 1-exo-acetamido-pyrrolizidine.

The title compound [systematic name: (1R*, 8S)-2-acetamidoocta-hydro-pyrrol-izin-4-ium chloride-N-[(1R, 8S)-hexa-hydro-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-acetamido-pyrrolizidine (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 mol-ecules 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)†à : 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.