Intramolecular Hydrogen Bonds in Amino Amides.

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Regarding the hydrogen bonded conformers of 3-aminopropionamide and -alanine, an obvious difference is that the former is the global minimum of the potential energy surface, whereas the latter is not. This is due to the energetically unfavourable trans-orientation of the COOH-group, which is a prerequisite for hydrogen bond formation in -alanine; the stabilization of the hydrogen bond is more than cancelled by the cis-trans-energy difference. The CONH-group, in contrast, has only one stable orientation, so the stabilization generated by the hydrogen bond comes to effect.

Beyond that obvious difference, the two H···N hydrogen bonds are quite different in strength: N-H···N in 3-aminopropionamide has a H···N-distance of 2.107 Å, which corresponds to 78% of the sum of the van der Waals radii, whereas O-H···N in -alanine has a H···N-distance of 1.864 Å (69%). Similarly, the H-N bond length increases by 0.006 Å in 3-aminopropionamide, but the O-H bond length increases by 0.017 Å in -alanine, and the lowest potential barrier of the H-bonded conformer is 12.1 kJ/mol in the amino amide but 28.7 kJ/mol in the amino acid.

It is also noteworthy to compare the internal rotation of the COR-group in the extended conformers. In the 3-aminopropionamide case, this reaction passes a single transition state after a rotation of 180° and then leads back to the original form. In the -alanine case with the sterically comparable trans-orientation of the COOH-group, three conformers are formed along this reaction for both, symmetrical and asymmetrical orientation of the amino group. The conformers along a full cycle of the symmetrical extended form are shown below this paragraph. This difference is not simply related to steric factors of the groups COOH and CONH, because the alterations of the relevant angles are identical within fractions of a degree: in the mirror symmetrical extended form of 3-aminopropionamide, e. g., the angle C-C-N is 115.74°, and in the transition state that corresponds to this form it is 118.22°; in the -alanine analogues the C-C-O angles are 115.77° and 118.39°, respectively.

The second intramolecular interaction, N-H···O=C, in contrast, is more or less identical in both, 3-aminopropionamide and -alanine. It is no hydrogen bond, but an attractive electrostatic interaction that has enough influence to avoid a third orientation of the amino group. In the case of 3-aminopropionamide it also has enough influence to couple the rotation of the groups NH and CONH to a complex pattern with three transition states, all of which toggle between the two conformers shown next to this paragraph.

3-aminopropionamide.
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