Intramolecular Hydrogen Bonds in Amino Alcohols.

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(global minimum) (2nd lowest minimum) (3rd lowest minimum) (4th lowest minimum) (5th lowest minimum) (6th lowest minimum)

5-Aminopentanol can form six different symmetry-unique conformers that contain an O-H···N hydrogen bond. Ball-and-stick models of these conformers are shown above this paragraph, in the order of their energies. As in the amino alcohols with a shorter carbon chain, they are lowest in energy; unlike in the smaller homologous systems, there is no significant energy gap between them and the following energy minima.

Similar to delta-aminopentanoic acid and delta-hydroxypentanoic acid, the various criteria for hydrogen bond strength do not give a consistent picture in the case of the O-H···N hydrogen bond in 5-aminopentanol: in the global minimum, for instance, it is not only the most stable according to energy but also according to the decrease in O-H vibration frequency; at the same time it is least stable according to the deviation of the O-H···N angle from linearity, and of intermediate strength according to H···N distance or increase of the O-H bond length. A plot of the various values, that describe H-bond strength, for the global minima of 2-aminoethanol, 3-aminopropanol, 4-aminobutanol, and 5-aminopentanol as a function of the ring size shows one common trend, though: an almost linear relationship for ring-sizes up to 7 (i.e. up to 4-aminobutanol), and a significant kink for the continuation to ring-size 8 (i.e. 5-aminopentanol).

Besides the conformers with an O-H···N hydrogen bond, there are numerous conformers with other attractive intramolecular interactions. Most of them also exhibit repulsive H···H interactions with distances around 2.2 Å.

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