Crystals of Nonpolar Molecules

In these crystals with relatively weak dispersive forces and strong short-range repulsion, one can make the assumption that the molecules wiU adopt the densest possible packing with the least possible repulsion. The arrangement of the molecules will be determined by atom-atom potentials. The lattice energy is minimised when the number of van der Waals atom-atom contacts is as large as possible. 

According to Kitaigorodskii [M6], the packing density can be characterised by a packing coefficient K  

The long-chain alkanes, in particular the linear hydrocarbons with n carbon atoms, are preferentially arranged in the solid state so that the CFl2 zig-zag chains of the individual molecules are parallel to each other, and thus form a layered structure with a parallel arrangement of the structural units (Fig. 2.9). Such an arrange- 

The aromatic compounds are of special interest. As an example, we show here the crystal structure of the very intensively investigated molecule anthracene (Fig. 2.10). The molecular structure, with the charge clouds of the n electrons perpendicular to the molecular plane (compare Fig. 1.2), shows in an understandable way that the polarisability is strongly anisotropic and has by far its largest value in the molecular plane. From the conditions of maximum dispersive interactions and optimum packing in space, the herringbone structure seen in Fig. 2.10 is most favourable in a monodinic crystal lattice with two molecules in the unit cell. Fig. 2.10 also shows the structiures and Table 2.3 the structural data for the other crystals of polyacene molecules, i.e. for naphthalene, tetracene and pentacene with 2, 4 or 5 aromatic rings. All of these substances crystallise like many others also in the same pattern. Table 2.3 also contains data about the orientations of the individual molecules in the unit cell for these crystals. 

From the stmctures shown in Fig. 2.10, one can see that the molecules are quite densely packed. Each molecule has ten nearest or next-nearest neighboiurs. Ind-dentally, note that the hydrogen atoms have been left out of the crystal structure drawings in order to make them clearer. 

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