Two or more species in the same crystal

Several types of crystals contain two or more types of molecules per unit cell, the hydrates providing a very simple example, in which the solvent molecules take part in the crystal structure and help satisfy requirements for hydrogen bonding and metal coordination. Sometimes solid solutions of two compounds with similar structures may form. This situation can cause problems in refining a crystal structure, especially if it is not real- 

TABLE 15.5. Potential parameters for polycyclic aromatic hydrocarbons, expressed in kJ mol and A. These may be used in Equation 15.6 (see Ref. 67). 

The types of structures containing more than one species that will be described here are tt- and charge-transfer complexes, clathrates and host-guest complexes. 

FIGURE 15.12. Some PAH-trinitrobenzene crystalline complexes, (a) 1,3,7,9-tetramethyluric acid and benzo[a]pyrene (Ref. 72). 

FIGURE 15.12 (cont d). (b) Crystal structure of 1,3,7,9-tetramethyluric acid and pyrene, (c) HOMO/LUMO overlaps for PAH-TNB complexes. Shown are signs of the wave functions (not electronic charge) for the HOMO and LUMO, and the overlap of molecules observed in the crystalline state (as predicted from the HOMO/LUMO overlap of positive with positive wave functions). 

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