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

20 J.P. Glusker
peptide plane and approximately near the C=O bond direction,so that the M◊◊◊O=C
angle is 140–170° for unidentate binding and 110–130° for bidentate binding. The
metal ion is rarely more than 35° from the peptide plane [36].
4.4
C-H◊◊◊O Interactions
The directionality of a hydrogen bond is partially a function of its strength – the
shorter the hydrogen bond the more likely the A-H◊◊◊B angle is to be near 180°.
Weaker interactions, such as C-H◊◊◊O or C-H◊◊◊N, described by Sutor [37], are
found when the hydrogen atom is attached to multiple bonds or strong electron
withdrawing groups (favoring the formation of H + ). These interactions, if
short or the order of the sum of the van der Waals radii, are generally attractive
rather than repulsive, and have come to be regarded as very weak hydrogen
bonds.
The interaction between a C-H group and a neighboring oxygen atom has
about one third the energy of an O-H◊◊◊O hydrogen bond. If, however, there is a
large number of such interactions, their contributions to the total energy of a
system may be substantial. For example, the importance of C-H◊◊◊O interactions
in aligning molecules has been shown by the design, by Desiraju and coworkers,
of dibenzylidene ketones (Fig. 13) that interact by way of C-H◊◊◊O
interactions with 1,3,5-trinitrobenzene [38]. Use was made of the great acidity
of the hydrogen atoms in 1,3,5-trinitrobenzene, and the propensity of nitro
groups to interact with hydrogen atoms bound to carbon atoms. It was shown by
X-ray crystallographic studies that there is a similarity of binding of dibenzylidene
acetone, cyclopentanone, and cyclohexanone. These complexes are analogous
to the designed complexes containing O-H◊◊◊O and N-H-O hydrogen
bonds (Fig. 13c), but in the complex with the dibenzylidene ketone described
here only C-H◊◊◊O interactions could be formed.
a
Fig. 13 a – c. The complex of a dibenzylidene ketone with 1,3,5-trinitrobenzene, showing
C-H◊◊◊O hydrogen bonds [38]: a chemical formula