Intramolecular Hydrogen Bonds in Hydroxy Acids.

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(global minimum) Glycolic acid has a potential energy surface in which many stationary points are mirror symmetrical. The global minimum, which is shown next to this paragraph, is among these too. It forms an intramolecular interaction O-H···O=C with a H···O-distance of 2.22 Å, which, according to the electron density between the aliphatic OH- and the CO-group, is not a hydrogen bond but of electrostatical nature. The global minimum is quite stable: the smallest potential barrier, which corresponds to an internal rotation of the aliphatic OH-group, is above 21 kJ/mol.

(2nd lowest energy minimum) The energetically second lowest conformer, which is shown next to this paragraph, is asymmetrical; the torsion angle O-Calpha-C=O is 154°, the torsion H-O-Calpha-C is -44°. This conformer forms an interaction O-H···(CO)O-H, which also is no hydrogen bond. The same characterization is also true for the mirror symmetrical transition state between this conformer and its mirror image, which has been mistaken for a local minimum in early studies [Ha et al., 1981].

(H-bonded form) A real hydrogen bond is formed in the energetically third conformer, which is shown next to this paragraph: the H···O-distance is 1.97 Å, which is 75% of the sum of the van der Waals-radii, and the O-H bond length is 0.0057 Å shorter than in the other conformers with a trans-orientation of the COOH group. Similarly, the calculated O-H vibration frequency is 83 cm^-1 lower. This conformer is considerably stabilized by the hydrogen bond: the smallest potential barrier is 40 kJ/mol.

An interesting trend can be gathered from the deviation (square root of sum of squares of differences) of the calculated rotation constants from the experimental values for the geometries of the global minimum, which have been optimized in RHF calculations with various basis sets. Basis sets with polarization functions perform even worse than minimal basis sets in this case.

basis   deviation  energy [a.u.]
STO-3G   0.2485    -298.63524
STO-6G   0.2066    -301.51535
3-21G    0.0154    -300.96339
4-31G    0.0816    -302.20754
6-31G    0.0341    -302.51371
6-31++G  0.0319    -302.52575 
6-311G   0.0785    -302.59875
6-31G*   0.3057    -302.65697
6-31G**  0.3170    -302.67352 
6-31+G** 0.3024    -302.68443  

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