Intramolecular Hydrogen Bonds in Amino Alcohols.

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(structure of the global minimum) 2-Aminoethanol can form two different interactions between the two functional groups: O-H···N and O···H-N. There is one symmetry-unique conformer with the former interaction, which is the global minimum of the potential energy surface. The geometry of this conformer, which is shown next to this paragraph, is of the envelope-type: the O-H···N-C sequence is almost co-planar, whereas the remaining carbon atom (C1) is located out of that plane. The interaction O-H···N is a hydrogen bond, according to all of the usual criteria, and it stabilizes this conformer (and its enantiomer) thermodynamically as well as kinetically: the conformer, which is second lowest in energy, has a relative energy of approximately 7 kJ/mol, and the lowest potential barrier, which happens to lead to this very conformer, is around 10.5 kJ/mol.

The O···H-N-interaction is present in four symmetry-unique conformers, which are shown below this paragraph. The O···H-N-interaction is not a hydrogen bond: the bond order is less than 0.01, the N-H bond lengths and vibrations frequencies do not deviate from the mean value of the other conformers, and there is no saddle point in the electron density between the atoms O and H. However, there is an electrostatic interaction, which prevents the formation of a third orientation of the amino group; it also causes these conformers to be slightly lower in energy than most of the other (extended) conformers, namely second, third, fifth, and sixth in energy.
(structure of conformer 2) (structure of conformer 3) (structure of conformer 5) (structure of conformer 6)

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