What is interpenetration

The large voids formed by some single nets are seldom empty. Besides solvent molecules and counter ions, these voids can be filled with one or more additional nets. To be classified as interpenetrating, these nets should not be able to separate without breaking any bonds. Self-penetration on the other hand is a topological property of a single net (see for example section 6.2.1) and not dealt with in this chapter. 

Depending on the constituting molecules the interpenetrated nets can be entwined together, located in the complimentary voids of the other net or a combination of both. Depending on the number of networks in the complete structure, they are classified as A -fold networks where N is two or more. Thus for a structure with four interpenetrating networks it will be a four-fold network. We also talk synonymously about degree of interpenetration. 

Currently, we note that the highest degree of interpenetration is the elevenfold hydrogen bonded diamond net of the 1 2 adduct of tetrakis(4-(3-hydroxyphenyl)phenyl)methane and benzoquinone, [9] and the ten fold diamond net in [Ag(l,12-dodecanedinitrile)2]N03 [10]. 

We may also distinguish between cases where the nets are maximally displaced from each other, and cases where there are attractive forces between the nets to give for example double helices or other kinds of interweaving [6]. 

We will discuss nomenclature of interpenetration at the end of this chapter because in order to understand the topology of interpenetration and thus the complete crystal structure we need a language to describe it with. However, we will first deal with strategies to avoid (or to promote) interpenetration. 

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