Planar network, definition

To have 32 symmetry classes for the external forms of crystals is a definite restriction, and it is obviously the consequence of inner structure. The translation periodicity limits the symmetry elements that may be present in a crystal. The most striking limitation is the absence of fivefold rotation in the world of crystals. Consider, for example, planar networks of regular polygons (Figure 9-16). Those with threefold, fourfold, and sixfold symmetry cover the... 

Whether or not fullerenes have to be considered aromatic has been debated since their discovery [48]. This is because the definition of aromaticity [116-118] is controversial and has changed many times over the last 175 years. The structure of fullerenes, especially that of the icosahedral representatives such as Cjq, implies their consideration as three-dimensional analogues of benzene and other planar aromatics. In contrast to such classical systems, however, the sp -networks of... 

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