Intramolecular Hydrogen Bonds in Amino Amides.

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The potential energy surface of 3-aminopropanionamide contains a total of six symmetry-unique conformers, which are shown below this paragraph in the order of their energy. This is an astonishing small number, since the "general wisdom" of organic structure chemistry predicts three possible orientations for internal rotations of the C-N and C-C bonds; for H_2N-CH_2-CH_2-CONH_2 this would mean 3³ = 27 local minima, i.e., 14 symmetry-unique conformers. One might expect that the difference between 14 conformers-to-be-expected and the six, which are actually observed, is caused by a number of strong intramolecular interactions. This is not the case, however: there are only two intramolecular interactions, namely N···H-N(CO), which is found in the global minimum, and N-H···O=C, which is present in the conformers of second the third lowest energy.

(all minima of aminopropionamide)

The hydrogen bond N···H-N(CO) in the global minimum has a H···N distance of 2.107 Å and a bond order of 0.056. This hydrogen bond is rather weak, according to all indicators (e.g., the hydrogen bond length is 78% of the sum of the van der Waals radii, and the H-N bond length increase is only 0.006 Å). Furthermore, the relative energies of all six conformers are more or less equally distributed in the range between 0 and 19.1 kJ/mol, which also indicates no major stabilization from this hydrogen bond. The second intramolecular interaction, N-H···O=C, is no hydrogen bond according to all of the usual criteria.

One of the mysteries of this potential energy surface with so few local minima is the internal rotation of the CONH_2-group in the extended conformers (fourth and fifth in energy): in both cases the energy increases steadily up to a transition state, in which this group is rotated by 180°; after passing the transition state, the path leads back to the original orientation. Slight shoulders appear in these energy profiles (see below), but without any tendency towards stationary points.

(asymmetric case) (symmetric case)
Energy profile of the internal rotation of the CONH_2-group in the extended forms with asymmetric (left or top) and symmetric (right or bottom) orientation of the amino group. Values are given in kJ/mol (relative to the global minimum) and degree (C-C-C=O), stationary points are marked.

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