Ab initio quantum-chemical study of conformational composition and intramolecular C-H center dot center dot center dot O interactions in 1-vinyl-2-formylimidazole and -benzimidazole

Ab initio calculation of the conformers of 1-vinyl-2-formylimidazole and -b enzimidazole showed that four conformers differing in the mutual orientatio n of the vinyl and formyl groups are possible. However, at room temperature these compounds exist virtually exclusively in the (N-vinyl, O)-cis form w ith trans orientation of the vinyl group relative to the formyl group. In t his conformation 1-vinyl-2-formylimidazole has a planar structure, and in 1 -vinyl-2-formylbenzimidazole the vinyl group appreciably deviates from the plane of the heteroring. The equilbrium distance between the alpha-hydrogen atom of the vinyl group and the oxygen atom is considerably shorter than t he sum of the van der Waals radii, which is consistent with weak intermolec ular hydrogen bonding C-H ... O.