Intramolecular Hydrogen Bonds in Amino Alcohols.

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Omega-amino-n-alkanoles and omega-amino acids both form intramolecular N···H-O hydrogen bonds, which are stronger than any other intramolecular interaction in these species. The direct comparison, which is given below using one of the criteria for such a hydrogen bond, shows that this bond is stronger in the omega-amino acids. It also shows that in both cases the various criteria for hydrogen bond strength agree only up to the seven-membered ring formed by the C_4-compound.

 (OH distances in amino alcohols)  (OH distances in amino acids)

O-H distances (Å) in N···H-O hydrogen bonded omega-amino alcohols (left or top) and omega-amino acids (right or bottom) as a function of the ring-size. The lines connect data for conformers of lowest energy.

(H-bonded aminoethanol) (H-bonded glycine) Another notable set of similarities and differences between omega-amino acids and omega-amino alcohols concerns the number and the geometry of conformers with the N···H-O hydrogen bond. Both 2-aminoethanol and glycine form only one symmetry-unique conformer, which is asymmetrical in the former but essentially symmetrical in the latter. The 2-aminoethanol potential energy surface contains two energetically equivalent reaction paths that connect the N···H-O bonded conformer with its enantiomer without breaking the hydrogen bond; these reaction paths do not pass the mirror-symmetrical conformation, in which the ring, that is closed by the hydrogen bond, becomes planar as in glycine, but remain chiral from beginning to end. In the case of the amino acids, this behaviour can be found in the potential energy surface of the next homologue, beta-alanine, which forms a six-membered ring. (H-bonded beta-alanine) There is a significant difference between these two cases, though: in beta-alanine, these reaction paths are the ones with the lowest energy barrier for the hydrogen bonded conformer, whereas in 2-aminoethanol several other reaction paths have a smaller barrier. Analogously, 3-aminopropanol shows a similar picture as gamma-aminobutyric acid: two pairs of N···H-O bonded enantiomers (A/A^m and B/B^m), which are connected in a H-bond conserving reaction cycle A-B-A^m-B^m-A. Again, this cycle has the lowest potential barriers in the amino acid but not in the amino alcohol case.

The comparison of 3-aminopropanol, beta-alanine, and 3-aminopropanal leads to the conclusion that the N···H-O hydrogen bonds, although they are the strongest of all hydrogen bonds formed in these systems, do not have the biggest impact on the various potential surfaces. Rather, the presence of the C=O group is the most influential factor. The amino alcohol / amino acid comparison given above leads to the same conclusion: the C=O group forces an essentially planar arrangement in glycine, which does not leave room for an enantiomer pair that is connected by chiral reaction paths. This can only happen with one an additional carbon atom, thus making the C_2 amino alcohol equivalent to the C_3 amino acid, and the C_3 amino alcohol equivalent to the C_4 amino acid. However, this equivalence of the number of H-bonded conformers and reaction paths is limited by the different strength of the N···H-O hydrogen bonds: the C_4 amino alcohol, in which N···H-O bond is strongest, is no longer an equivalent of the C_5 amino acid. The different strength of the N···H-O hydrogen bonds actually is a consequence of the C=O group: it makes the hydrogen atom of the O-H group much more positive than in an alcohol, and thus the H-bond stronger.

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