Intramolecular Hydrogen Bonds in Hydroxy Acids.

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(structures A-F)

The potential energy surface of delta-hydroxypentanoic acid contains six symmetry-unique local minima with a hydrogen bond (CO)O-H···O-H. The geometries of these minima (A, B, ... F) are shown on top of this paragraph in the order of their energies. They are connected among themselves and their mirror images (a, b, ..., f) by a network of hydrogen bond preserving reactions, as shown in the following scheme.

Two of these reactions deserve special attention. The first of these two was found to have a barrier of only 0.16 kJ/mol for the conversion of D to B. These two minima differ essentially in the orientation of the aliphatic OH-group and have an energy difference of 1.23 kJ/mol. In D, the unscaled harmonic vibration frequency related to the internal rotation of the OH-group (211.24 cm^-1) equals a vibrational zero-point energy of 1.27 kJ/mol. This clearly exceeds the rotational barrier, hence D is no stable conformer although it is a true local minimum of the potential energy surface. A similar situation exists for E and F, which also differ in the orientation of the OH-group only. F has a barrier of 1.03 kJ/mol in the reaction to E, but a vibrational zero-point energy of 1.77 kJ/mol for the vibration frequency associated with this reaction (295.54 cm^-1). Thus the same conclusion can be drawn for F.

In contrast to these instabilities, conformer A is of considerable stability: this can not only be seen from the large energy gap between A and B (11.62 kJ/mol), but also from the potential barriers for H-bond breaking reactions (39.5-50.1 kJ/mol) as well as for the hydrogen bond preserving ones (28.4-44.5 kJ/mol).

These differences in energetic stability are astonishing in view of the fact that all six energy minima give a very consistent picture in terms of bond distances: the interatomic O···H distance is between 65% and 68% of the sum of the van der Waals radii in all cases, and the H-bond formation leads to an elongation of the acidic O-H and the aliphatic C-O bond of approximately 0.011-0.013 Å (1.2-1.3%) and 0.007-0.012 Å (0.5-0.8%), respectively. Similar to delta-aminopentanoic acid and 5-amino pentanol, the various criteria for hydrogen bond strength give only a diffuse picture. Some of them (O···H-distance, elongation of acidic O-H bond, and electron density of the H-bond) even indicate the kinetically instable non-conformer D to have the strongest hydrogen bond.

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