21-Crown-7 is an organic compound with the formula [C2H4O]7 and the IUPAC name 1,4,7,10,13,16,19-heptaoxacycloheneicosane.

Similar to other crown ethers, 21-crown-7 functions as a ligand for certain metal cations, with a specific affinity for caesium cations.

[1] The dipole moment of 21-crown-7 varies depending on the solvent and temperatures.

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