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

Directional Aspects of Intermolecular Interactions 25
Fig. 16. The different styles of packing in the crystalline state of some aminophenols [44–46].
It is shown that p-aminophenol utilizes O-H◊◊◊N and N-H◊◊◊O hydrogen bonds in crystal
packing, while the o- and m-compounds and 2-amino-4-methylphenol also utilize C-H◊◊◊O
and N-H◊◊◊p interactions
molecules containing electron-poor ring systems. The resulting complexes,
which involve interactions between the p-electron systems of the two types of
molecules, are generally called p-complexes. They are characterized by short
intermolecular distances perpendicular to the stacking direction, like graphite.
Therefore it is common to find crystals growing as needles, elongated along the
stacking direction. Examples of these types of complexes are provided by the
trinitrobenzene complexes of PAHs, diagrammed in Fig. 17 [52, 53].
The two molecules forming such a p-complex consist of a donor molecule
with a low ionization potential so that an electron can be readily lost (a delocalized
p-electron of the PAH) and an acceptor molecule with a high affinity for
electrons (the aromatic ring of trinitrobenzene which is electron-poor because
the three nitro groups pull electrons out of it). As a result, stacks of alternating
donor and acceptor molecules are found in the crystal. The relative orientations
within the parallel planes of these donor and acceptor molecules are determined,to
a considerable extent,by the orientations of charge distributions of the