Rigidity, Side Arms, and Donor Atoms

Macrocycles of the rigid type (e.g., small cryptands and other preorganized macrocycles) discriminate between cations that are either smaller or larger than the one that exactly fits into the cavity (peak selectivity). Macrocycles of the flexible type (e.g., larger polyether crowns and cryptands) discriminate principally among smaller cations (plateau selectivity) (Vogtle and Weber, 1989). 

The number, kind, and arrangement of donor atoms also play important roles in macrocycle selectivities. Oxygen donor atoms in classical crown ethers have the largest affinities for alkali, alkaline-earth, and lanthanide cations  

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