Fig. 13.11 If zinc chloride is reacted with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (top center), crystals of a porous structure with cavities are formed (a). A drop of solvent, in which an organic test molecule to be investigated is dissolved, can be allowed to diffuse into the huge cavities of this crystal lattice. If the process occurs slowly and in thermodynamic equilibrium, the test compound places itself in an regular fashion in the empty cavities of the lattice (b). Its geometry can then be determined together with the lattice of the crystalline sponge (c). Only very small amounts of test substance are required for the method. For example, substances formed during drug metabolism can be characterized after their chromatographic separation (d). The molecule shown in (c) is the glucuronidated metabolite of gemfibrozil.