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

122 R.E. Meléndez · A.D. Hamilton
3.4
Weak Interactions
In addition to strong hydrogen bonds, the use of weak intermolecular interactions
has been predicted to yield new and exciting results in the future of crystal
engineering. Hydrogen bonds of type C-H◊◊◊O(N) and O(N)-H◊◊◊p as well as
polarization induced iodo◊◊◊nitro interactions have been examined and used in
designing new solids [68].
The study of C-H◊◊◊O hydrogen bonds has become increasingly important in
our understanding of the packing of molecules in crystals [69]. In the past,
observation of higher boiling [70] and melting [71] points in organic compounds
have been ascribed to weak C-H◊◊◊O hydrogen bond formation. In a
survey of 113 published neutron diffraction organic crystal structures, Taylor
and Kennard [72] concluded that C-H◊◊◊O, C-H◊◊◊N, and C-H◊◊◊Cl interactions
are more likely to be attractive than repulsive. In their study, typical C◊◊◊O(N)
distances occur in the range of 3–4 Å and the hydrogen bond angles are observed
between 100–180°, but with a major cluster around 150–160° (see also
the articles by J.P. Glusker and A.Nangia and G.R. Desiraju in this volume).
Desiraju has identified a series of recognition patterns which possess similar
symmetry and directionality. The similarity in the formation of tapes by piperazine-2,5-dione
(39), 1,4-benzoquinone (40) and 1,4-dicyanobenzene (41) can
be observed in Fig. 28. The knowledge of the structure of 39 stabilized by
N-H◊◊◊O hydrogen bonds can serve as a guide in anticipating the formation of
tapes by 40 and 41. In the latter examples the directional properties of C-H◊◊◊O
and C-H◊◊◊N hydrogen bonds are evident [68].
39
40
41
Fig. 28. Tape structures formed by 39, 40 and 41