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.
is not part
of this H-bond conserving reaction cycle (for which
3-aminopropanol.
5-aminopentanol.
Quantum Chemistry Group.