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The X-ray molecular structure of two benzimidazoles unsubstituted on the nitrogen atoms (2-ethyl-1 H -benzimidazole and 2-propyl-1 H -benzimidazole) has been determined and analysed in the context of crystallographic and solid-state NMR data of seven substituted benzimidazoles. The first compound presents polymorphism, and two polymorphs [forms  3a  and  3c ] were obtained from different solutions of dichloromethane/hexane. Polymorph ( 3a ) contains two independent molecules differing in the conformation of the ethyl group. Polymorph ( 3c ) contains only one molecule in the asymmetric unit, with similar configuration as the ordered molecule of polymorph ( 3a ). Another polymorphic form ( 3b ) is obtained by cooling form ( 3a ) below 179 K. The transition is related to an order-disorder process and involves a change in the hydrogen-bonding scheme. The second compound ( 4 ) presents enantiomorphism and its asymmetric unit contains four independent molecules that differ in the conformation of the 2-propyl group. For form ( 3a ), the splitting of NMR signals is almost perfectly consistent with the features of the crystal structure, whereas not splitting is observed for compound ( 4 ). Agreements between NMR and X-ray crystallography techniques are also observed in other simple 2-R-1 H -benzimidazoles (R = C 4 H 9 , CH 2 C 6 H 5  and C 6 H 5 ), but disagreements are found for R = H, CH 3 .

Crystal and molecular structures of two 1 H -2-substituted benzimidazoles