Crystals with a Low Packing Density, Clathrates

There are also molecules whose crystals contain extended one-dimensional holes (channels) or two-dimensional cavities (planar cavities) in which other molecides can be trapped without covalent bonding, i.e. by van der Waals bonding alone. These are the inclusion crystals or clathrates. 

A typical example of them is urea, H2N - C-NH2 (Fig. 2.15). Urea crystallises 

An example of crystals with planar inclusions are the clathrates of tetraphenyl-porphyrin (Fig. 2.16). Due to the steric requirements of the porphyrin molecule 

Further examples of clathrates, clathrate formers and porous molecular structures can be found in Vogtle [M2] and also in [7]. 

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