Intramolecular Hydrogen Bonds in Amino Amides.

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(global minimum of 3-aminopropionamide) (H-bonded beta-alanine) Regarding the hydrogen bonded conformers of 3-aminopropionamide and beta-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 beta-alanine; the stabilization of the hydrogen bond is more than cancelled by the cis-trans-energy difference. The CONH_2-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 beta-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 beta-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.

(extended form of 3-aminopropionamide) 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 beta-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_2, 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 beta-alanine analogues the C-C-O angles are 115.77° and 118.39°, respectively.

(extended beta-alanine, COOH rotation)

(3-aminopropionamide, structure2) (3-aminopropionamide, structure3) The second intramolecular interaction, N-H···O=C, in contrast, is more or less identical in both, 3-aminopropionamide and beta-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_2 and CONH_2 to a complex pattern with three transition states, all of which toggle between the two conformers shown next to this paragraph.

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