Intramolecular Hydrogen Bonds in Amino Alcohols.

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(global minimum) (2nd lowest minimum) (3rd lowest minimum)

4-Aminobutanol can form three different symmetry-unique conformers that contain an O-H···N hydrogen bond. These conformers are significantly lower in energy than all other and are shown on top of this paragraph. The geometry of the global minimum, which is shown on the left side, is a kind of distorted chair form. It is kinetically more stable than its analogue in the 3-aminopropanol surface, with a lowest potential barrier around 19 kJ/mol.

Again, there are reaction paths that preserve the hydrogen bond and link the three H-bonded forms in a closed cycle I-II-III-Im-IIm-IIIm-I (the superscript "m" denoting the mirror image). As in 3-aminopropanol, the reaction path of the global minimum, which has the lowest barrier, is not part of this H-bond conserving reaction cycle (for which the barriers are 20.6 and 26.1 kJ/mol).

In total, the potential energy surface of 4-aminobutanol contains 110 symmetry-unique conformers. Among these, there are nine with a N-H···O hydrogen bond, that form seven-membered rings, five with a C-H···N hydrogen bond, that closes six-membered rings, and seven with a C-H···O hydrogen bond, which also form six-membered rings. All of these interactions are true hydrogen bonds in the sense that they contain a critical point in the electron density between the respective atom pair. However, the electron density in this point is low in all of these cases, and, in contrast to the O-H···N hydrogen bond that is present in the three energetically most stable conformers, neither bond lengths nor vibration frequencies are influenced.

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