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

Directional Aspects of Intermolecular Interactions 9
carbon atoms, pairs of hydrogen atoms, and individual pairs of carbon and
hydrogen atoms. The dominant intermolecular interactions in crystals of the
smaller PAHs are between hydrogen atoms and carbon atoms.
Delocalized electrons on the carbon atoms of benzene, and slight residual
positive charges on the hydrogen atoms, result in the packing arrangement
shown in Fig. 2 for crystalline benzene. The hydrogen atoms are attracted to the
more electron-rich carbon atoms to give a herring-bone arrangement of molecules.
As the PAH becomes larger, the carbon-to-hydrogen ratio increases and
intermolecular carbon-carbon interactions become relatively more important.
The result is that large PAH molecules stack one above the other [12, 13].
Equation (1) makes it possible to predict packing arrangements for PAHs. In
particular, unit cell dimensions can be predicted for a crystalline PAH and their
values can be compared with experimental values [11].
a
b
Fig. 2 a, b. Packing of molecules in crystalline benzene, showing: a a stereoview of the overall
structure; b the shortest C◊◊◊H distances in Å (representative of an H◊◊◊p interactions) [109]