Spatial arrangement of molecules in homomolecular Z' = 2 structures.

The Box Model of crystal packing describes unit cells in terms of a limited number of arrangements of molecular building blocks. An analysis of Z' < or = 1 structures has shown that cell dimensions are related to molecular dimensions in a systematic way and that the spatial arrangement of molecules in crystal structures is very similar, irrespective of Z or space group. In this paper it is shown that the spatial arrangement of molecules in Z' = 2 structures are, within the context of the Box Model, very similar to that found for Z' < or = 1 structures. The absence of crystallographic symmetry does not appear to affect correlations between molecular dimensions and cell dimensions, or between the packing patterns and the positions of molecules in the unit cell, established from the analysis of Z' < or = 1 structures. The preference shown by Z' = 2 structures for low surface-area packing patterns and the observation that strong energetic interactions are most often found between the large faces of the independent molecules reaffirms the importance of molecular shape in crystal packing.