Intramolecular Hydrogen Bonds in Amino Aldehydes.

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(repulsive 3-aminopropanal conformer) Evaluation of all intramolecular interactions in 3-aminopropanal, beta-alanine, and 3-aminopropanol shows that the hydrogen bond N···H-O is the strongest interaction in all of these compounds. Consequently all conformers, in which this interaction is present, are of considerable kinetical stability: the lowest potential barriers are 28.66 kJ/mol (beta-alanine) and 14.84 kJ/mol (3-aminopropanol).

(H-bonded beta-alanine) The expectation that this strongest interaction has the biggest impact on the potential energy surface (PES) is, however, misleading. This is already obvious from the geometries of those conformers, in which it is present, as discussed in the previous page of this tour. Hence it may be concluded that the CO group rather than the hydrogen bond N···H-O has the biggest impact on the PES of those compounds, in which it is present. This conclusion, which also emerges from the comparison of the series of homologous omega-amino acids and omega-amino alcohols, also explains the similarity of the PES of 3-aminopropanal and beta-alanine. This similarity is not only obvious from the geometries and the electron densities of the global minima, but also from the geometries of all other conformers. For the beta-alanine conformer with the N···H-O hydrogen bond and its 3-aminopropanal match this similarity of geometries is impressive in view of the fact, that the electronic interaction repulsive in the latter (causing, e.g., completely different reactions of the two conformers upon rotation of the -CO(R) group).

(H-bonded form of 3-aminopropanol) The conclusion, that the CO group influences the PES more than any other structural feature, is also supported by the fact, that the OH group has three stable orientations relative to the rest of the molecule in 3-aminopropanol (or any other alcohol) but only two in beta-alanine (or any other carbon acid) due to the strong electrostatic interaction between the groups CO and OH.

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