Intramolecular Hydrogen Bonds in Amino Aldehydes.

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(global minimum of beta-alanine) (global minimum of 3-aminopropanal) The comparison between 3-aminopropanal and beta-alanine reveals a surprising amount of similarity between the geometries of the various conformers. The two global minima, for instance, which are shown next to this paragraph, are virtually identical except for the intended difference that the OH-group of the acid is replaced by a hydrogen atom in the aldehyde. This similarity also includes the N-H···O=C interaction, which in both cases is an attractive electrostatic interaction but no hydrogen bond.

(H-bonded beta-alanine) (repulsive 3-aminopropanal minimum) Astonishingly, this similarity even extends to the conformers with the strongest intramolecular interactions, which are displayed next to this paragraph. They are the conformer with the electronically repulsive C-H···N interaction in the 3-aminopropanal case and the conformer with the highly attractive intramolecular N···H-O hydrogen bond in the beta-alanine case. Despite the totally different nature of these two interactions, both conformers exhibit an identical orientation of the molecular backbone.

Furthermore, the relative energies of the matching conformers run parallel too, except for two beta-alanine conformers, which differ in energy only by 0.017 kJ/mol.

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