Infrared spectroscopic study of the conformations assumed, in solution, by β‐diamides considered as model compounds for the repeating unit in poly amides synthesized from β‐amino and β‐imino acids

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AbstractIn order to get a closer understanding of the conformational features displayed by poly‐amides obtained from different β‐amino and β‐imino acids, a systematic investigation was carried out with three sets of model β‐diamides. equation image Very dilute solutions in carbon tetrachloride were studied by infrared spectroscopy in the 3200‐3500 cm−1 frequency range which includes the vNH stretching absorption bands.Depending on the substitution of the middle aliphatic sequence, molecules I (R, R') mainly assume either a stretched form (when R = H in β‐alanine derivatives) or a folded six‐membered ring conformation (when R = Me in the compounds synthesized from β‐aminobutyric acid). This latter conformer is stabilized by an intramolecular hydrogen bonding.A similar behavior was evidenced with N‐dimethylated species III(R). The β‐alanine derivative III(H) was found to be in an open form, whereas, the β‐aminobutyric analogous diamide III(Me) assumed a folded structure. In this latter case, however, it was shown that the folding is mainly governed by a NH…π interaction between the NH group and the π orbital of the tertiary amide function.On the other hand, the N‐methylation of the acetylamino group in models II(R) produces preferentially an eight‐membered ring chelate which is stabilized by an intramolecular hydrogen bonding involving the NH of the C‐terminal amide function and the highly nucleophilic CO in the N‐methylated acetylamino group.Each of the evidenced conformers is described by a set of three rotational angles and is experimentally characterized by its specific spectroscopic features.