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

The title compound [systematic name: (1*, 8)-2-acetamidoocta-hydro-pyrrol-izin-4-ium chloride--[(1, 8)-hexa-hydro-1-pyrrolizin-2-yl)acetamide (1/1)], 2(CHNO)·HCl or CHNO·Cl·CHNO, arose as an unexpected product when 1--acetamido-pyrrolizidine (AcAP; CHNO) was dissolved in CHCl. 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) 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 ( N-H⋯N + N⋯H-N). Computational studies suggest that the disorder model is slightly more stable, but the energy difference is very small.