198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Supramolecular Synthons and Pattern Recognition 89
not rigorously quantifiable. In this respect, the usage of the synthon concept in
crystal engineering and supramolecular chemistry follows very closely its usage
in classical organic synthesis. In both these usages, simplification is combined
with chemical focus. Given that crystal structures need to be simplified before
they can be compared and analysed, the graph set notation doubtless offers an
accurate topological description of hydrogen bonded patterns. However, the
simplification is drastic and is achieved at the cost of obscuring the chemical
nature of the recognition events that are the primary causes of crystallisation.
On balance, synthons appear to offer a middle ground wherein simplification is
obtained without compromising the chemical information contained in the
supramolecular system [83–90].
A common deficiency of the graph set and synthon approaches to crystal
structure description is that they represent interactions in a crystal without
any indication as to whether they are strong or weak and as to their importance
in controlling crystal packing. Such information is available in a new pictorial
method called NIPMAT (Nonbonded Interaction Pattern MATrix) that is
used to display intermolecular interactions in crystals [91]. A two-dimensional
quantitative representation gives a single-view visualisation of the relative
strengths of the hydrogen bonding and van der Waals forces operating in the
crystal. For example, the crystal structures of 2-, 3- and 4-aminophenols detailed
earlier in this article may now be compared through their NIPMAT plots
[92]. Examination of Fig. 10 shows that hydrogen bonding between the amino
and hydroxy groups is the common, dominant intermolecular interaction as
evidenced from the dark grey squares for H6-O1 and H7-N1 in the upper right
hand corner of the plot. The contribution from the N-H◊ ◊ ◊p interaction in
2- and 3-aminophenols is revealed by the grey squares for H5-C3/4/5 and
H5-C4/5/6 respectively (Figs. 10a, b), whereas this interaction is absent in
4-aminophenol (Fig. 10c).
a
NIPMAT plot of 2-aminophenol 28
Fig. 10 a – c. Comparison of crystal structures with NIPMAT plots: a 2-aminophenol 28