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

Abstract

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.