Intramolecular Hydrogen Bonds in Amino Alcohols.

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(global minimum) (minimum #2) 3-Aminopropanol can form two different symmetry-unique conformers that contain an O-H···N hydrogen bond. These are the minima of lowest and second lowest energy in the potential energy surface. The geometry of the global minimum, which is shown on the left side, is a distorted form of the well-known cyclohexane chair form. Similarly, the geometry of the second lowest energy is a distorted boat form, which is shown on the right side. As is can be seen already from the overlap of the respective spheres, the hydrogen bond in the global minimum (and its enantiomer) is much stronger. The global minima are of considerable stability due to this hydrogen bond: the conformer next in energy has a relative energy of almost 8 kJ/mol, and the lowest potential barrier of the global minima is around 15 kJ/mol. (In total, the 3-aminopropanol potential energy surface contains 36 symmetry unique local minima.)  

Similar to gamma-aminobutyric acid, there are reaction paths that preserve the hydrogen bond and link the two H-bonded forms in a closed cycle I-II-I^m-II^m-I (the superscript "m" denoting the mirror image). Interestingly, for the global minimum of 3-aminopropanol the reaction path with with lowest barrier is not one of these H-bond conserving reactions (which have barriers of 16.4 and 20.0 kJ/mol).

(minimum #4) (minimum #7) (minimum #9) (minimum #18)

Similar to aminoethanol, there are four symmetry-unique conformers with a N-H···O interaction, that have the same (all-gauche) orientation of the heavy atom framework and two different orientations of both, the OH- and the NH_2-group. These four conformers are shown on top of this paragraph, with energy increasing from left to right. Unlike in ethanolamine, these N-H···O interactions are hydrogen bonds, but not strong enough to have any influence on bond lengths, distances, or energies. It does, however, exclude a third orientation of the amino group.

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