The comparison between
3-aminopropanal and
3-aminopropanol is relatively easy: there is
no similarity at all, as it may be expected from the different
functional groups.
The geometries of the
global minima, which are shown next to this paragraph,
may be taken as examples: the aldehyde has all carbon atoms and the
CHO-group in one plane, from which the amino group sticks out;
the alcohol, in contrast, is in a distorted chair form, with the
O-H···N hydrogen bond closing the six-membered ring.
The comparison between the hydrogen bonded forms of
3-aminopropanol
and -alanine (shown in the graphic insert of this
paragraph) is more worthwile: both form six-membered rings,
but that of -alanine is
of the envelope-type.
The hydrogen bond is considerably stronger in -alanine than
in 3-aminopropanol, which is well visualized by the overlap in the
fused sphere models. Part of this difference in strength can be
ascribed to the fact that the hydrogen atom of the OH-group
is much more positively charged in -alanine than in 3-aminopropanol.
In terms of electron density along the hydrogen bond,
the strength of this bond in -alanine is more than twice the one of
3-aminopropanol. The weaker nature of the hydrogen bond in that latter
is also manifested
by the existence of a second conformer with
the same interaction, the geometry of which is a distorted boat form.
The comparison between
3-aminopropanal and
3-aminopropanol is relatively easy: there is
no similarity at all, as it may be expected from the different
functional groups.
The geometries of the
global minima, which are shown next to this paragraph,
may be taken as examples: the aldehyde has all carbon atoms and the
CHO-group in one plane, from which the amino group sticks out;
the alcohol, in contrast, is in a distorted chair form, with the
O-H···N hydrogen bond closing the six-membered ring.
The comparison between the hydrogen bonded forms of
3-aminopropanol
and 
-alanine (shown in the graphic insert of this
paragraph) is more worthwile: both form six-membered rings,
but that of 
-alanine is
of the envelope-type.
The hydrogen bond is considerably stronger in 
comparison with 
conclusions.
Quantum Chemistry Group.