Intramolecular Hydrogen Bonds in Hydroxy Acids.

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(global minimum) Beta-hydroxypropionic acid basically forms the same intramolecular interactions as glycolic acid: O-H···O=C, O-H···(CO)O-H, and (CO)O-H···O-H. The first of these interactions occurs in the global minimum, the geometry of which is shown next to this paragraph. In this conformer, the COOH group is coplanar with the plane of the three carbon atoms, and the aliphatic OH group sticks out of that plane and points towards the carbonyl oxygen atom. In the resulting interaction, the H···O-distance is slightly larger (2.30 Å) than in the corresponding conformer of glycolic acid. According to an analysis of the electron density, this interaction is not a hydrogen bond but electrostatic in nature.

(2nd energy minimum) The geometry of the conformer of second lowest energy, which is shown next to this paragraph, is characterized by a COOH group that is coplanar with the aliphatic OH group. The carbon atoms alpha and beta are located above and below this plane, respectively. The O-H···O-H-interaction in this conformer is a true hydrogen bond, which is weak, however: the H···O distance is 2.15 Å and the bond order is 0.023. Hence, there is no stabilization associated with this H-bond: the relative energy of this conformer is 7.2 kJ/mol.

(H-bonded form) The (CO)O-H···O-H-interaction is a much stronger hydrogen bond with a bond order of 0.062 and a H···O-distance of 1.84 Å; this value is significantly lower than the one in the corresponding conformer of glycolic acid (1.97 Å), which would indicate a stronger H-bond. The kinetic stability, however, is lower than in glycolic acid: the smallest potential barrier is 27 kJ/mol and can be found in two different reaction paths (with almost identical values). The first of these reaction paths corresponds to the conversion of this conformer to its mirror image, the second corresponds to a combined rotation of the COOH- and the aliphatic OH-group and results in a conformer that is identical with the global minimum, except for the trans-orientation of the COOH group. The potential energy surface of beta-hydroxypropionic acid contains a total of 16 symmetry-unique conformers. Seven out of these 16 are lower in energy than this H-bonded conformer (Erel=16.7 kJ/mol), which is another indication for the minor stabilization due to the hydrogen bond. A numerical estimate confirms this by yielding a stabilization energy of 36.6 kJ/mol for beta-hydroxypropionic acid, but 54.4 kJ/mol for glycolic acid.

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