Azetidine, pyrrolidine and hexamethyleneimine at 170 K

The crystal structures of the cyclic amines azetidine (C(3)H(7)N), pyrrolidine (C(4)H(9)N) and hexamethyleneimine (homopiperidine, C(6)H(13)N), of the series (CH(2))(n)NH, with n = 3, 4 and 6, respectively, have been determined at 170 K, following in situ crystallization from the melt. These structures provide crystallographic data to complete the homologous series of cyclic amines (CH(2))(n)NH, for n = 2-6. Azetidine and pyrrolidine contain chains propagating along 2(1) screw axes, in which the molecules are linked by co-operative N-H...N hydrogen bonds. Azetidine has two molecules in its asymmetric unit, while pyrrolidine has only one. Hexamethyleneimine contains tetrameric hydrogen-bonded rings formed about crystallographic inversion centres, with two molecules in its asymmetric unit. The observation of crystallographically distinct molecules in the hydrogen-bonded chains of azetidine and cyclic hydrogen-bonded motifs in hexamethyleneimine is consistent with expectations derived from comparison with monoalcohols forming chains or rings by co-operative O-H...O hydrogen bonds. The next member of the cyclic amine series, heptamethyleneimine, forms a cubic plastic phase on cooling from the melt.