Other Packing Considerations

An alternative possibility is a vertical stacking structure where the p-strands are arranged on top of each other, again satisfying electrostatic and hydrophobic packing considerations (Fig. 20). Whichever is more likely, this is still far from a complete picture because the exact arrangement of the two disulphide-linked peptide chains is not yet known. 

Not all structures are based on close packed lattices. Ions that are large and soft often adopt structures based on a primitive or body centred cubic lattice as found in CsCl (22173) and a-AgI (200108). Others, such as perovskite, ABO3 (Fig. 10.4), are based on close packed lattices that comprise both anions and large cations. The larger and softer the ions, the more variations appear, but the lattice packing principle can still be used. Santoro et al. (1999,2000) have shown how close-packing considerations combined with the use of bond valences can give a quantitative prediction of the structure of BaRuOs (10253). 

Leaving aside this dynamical problem, we can make some further remarks about the equilibrium structure of Van der Waals molecules. Some attempts have been made to predict this structure from the molecular properties, multipole moments, polarizabilities, which are reflected in the long range interactions (electrostatic, dispersion). Other authors have assumed that the equilibrium structure of Van der Waals dimers resembles the structure of nearest neighbour pairs in molecular crystals. The latter approach could possibly be justified by packing considerations (short range repulsion). An example of the first approach is the prediction of a T-shaped (0 = 90°, 0 = < ) = 0°) equilibrium structure for the Nj-dimer, mainly... 

Electrostatic effects are undoubtedly of importance in molecular crystals, but, as heats of sublimation show, they are small and packing considerations must be put first. When electrostatic effects arc weak, owing to high symmetry of the interacting systems, other forces must be sought. 

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