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

24 J.P. Glusker
rine-containing compounds leads to a better understanding of hydrogen bonding
in general.
An interaction between a C-F bond and a metal ion (C-F◊◊◊M n+ ) has been
observed so far by X-ray diffraction studies only when the metal ion is an alkali
metal ion [40]. The coordination of carbon-bound oxygen and fluorine atoms
around the metal ion can be analyzed in terms of the requirement that the sum
of the bond valences should be near 1.0 for a monovalent cation [43]. This sum
is only achieved if the M n+ ◊◊◊F-C interaction is taken into account as well as those
involving oxygen atoms. The fluorine atom contributes to the local neutralization
of the charge of the cation, and is, indeed, a significant member of the first
coordination sphere of the metal ion.
4.6
H◊◊◊ Interactions
Strong O-H◊◊◊O and N-H◊◊◊O hydrogen bonds can be used in the design of
structures because they generally dominate packing if they can be formed.
Other types of interactions, however, need also to be taken into account. An
excellent example of this is provided by a neutron diffraction study of 2-, 3-, and
4-aminophenols [44–46]. In these crystal structures, studied by neutron diffraction,
there are significant N-H◊◊◊p interactions in which the hydrogen atom
points to the electron-rich aromatic ring, implying the importance of herringbone
interactions of the type described earlier for benzene and PAHs. Each
nitrogen and oxygen atom in these structures is surrounded by three groups,
which, including the bond to the carbon atom of the molecule, leads to a tetrahedral
bonding situation – but not one that would have been readily predicted.
This is shown in Fig. 16. Thus there are two important types of interactions
involving delocalized p-electron systems, as described earlier. These are the
C-H◊◊◊p or C-H◊◊◊C interactions (where p may be any part of an electron-rich
aromatic system, a delocalized double bond or a triple carbon-carbon bond)
which give rise to a “herring-bone” arrangement of molecules, as shown for
benzene in Fig. 2.
O-H◊◊◊p interactions are also seen in 2- and 3-aminophenol crystals in 2ethynyladamantan-2-ol
[47], and in several protein structures in the PDB. In one
(O-glycosyl hydrolase, with bound 4-acetylamino-3-amino benzoic acid, 1IVE,
[48]) an O-H◊◊◊p interaction is envisioned between Tyr 406 and Asp 151. In the
other (m-glutathione S-transferase, 1GST, [49]) an O-H-p bond to Tyr 6 is essential
for glutathione binding. Interactions between indole rings and quaternary
nitrogen atoms have also been described [24, 50]. The positively charged nitrogen
stacks over the indole ring system. The binding of tacrine (tetrahydro-9aminoacridine),
an anti-Alzheimer drug, to acetylcholinesterase shows a similar
interaction [51].
The other type of interaction, a p-p interaction, is found in crystals of PAHs;
these are rich in delocalized electrons and mainly consist of sp 2 carbon atoms
with peripheral hydrogen atoms attached. Their structures resemble that of
graphite in which planar hexagonally disposed layers of carbon atoms are separated
by about 4 Å. Large PAHs also form molecular complexes with other flat