Intramolecular Hydrogen Bonds in Amino Acids.

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(structure of H-bonded form) Glycine forms one conformer with a O-H···N hydrogen bond. With split-valence basis sets, this conformer is mirror symmetrical, whereas basis sets with polarization functions yield a similar, but slightly unsymmetrical geometry. Therefore, in the latter case there are strictly speaking two H-bonded local minima in the potential energy surface, but the barrier between these two is extremely low and vibrational averaging takes place soon.

The H-bonded conformer is not the global minimum of the potential energy surface, which has made history in the field of quantum chemistry: it was first identified from the gas-phase microwave spectrum as the most stable form, because of the high intensity of its signals, whereas the ab initio results indicated a mirror symmetrical conformation without hydrogen bonds as the global minimum.

(structure of global minimum) Based on the predictions for the rotational constants of this form, this conformer could then also be identified in the microwave spectrum. The apparant difference between theoretical and experimental results found a simple explanation: the higher microwave intensity of the H-bonded form is not caused by higher population, which means lower energy, but by the significantly higher dipole moment that is due to the hydrogen bond. Since this episode, glycine has not only been the topic of several investigations, but also served as a test molecule for new quantum chemical techniques.

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