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Exceptionally high selectivity for cesium ions (α(Cs/Na) > 33000) is displayed by calix[4]arenecrowns‐6 in the 1,3‐alternate conformation. They also remove 137Cs quantitatively (>96%) from radioactive waste that is 1 M in HNO3. The complexation properties result from the simultaneous operation of several effects: the size of the crown ether ring, the polarity of the calix conformation, and the strength of the cation/π‐electron interactions. The latter interaction was evident in the X‐ray crystal structure of the cesium complex (shown on the right).

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Dynamic 'H NMR measurements of the tetramethyl ether of p-tert-butylcalix[4]arene (2) show for the first time that all four possible conformations of one particular calix[4]arene are present, including the 1 ,balternate conformation. The thermodynamically most stable partial cone conformation readily interconverts to a cone or to a 1,3-alternate conformation; the interconversion to a 1,2-alternate conformation is much slower. The 1,2-alternate conformation of 2 is the kinetically stable conformation at the I H NMR time scale. The 1,a-alternate conformation was confirmed by comparison of its 'H NMR spectrum with that of the newly synthesized tetraethyl ether of p-tert-butylcalix[4]arene in a fixed 1,2-alternate conformation (6), of which the X-ray structure was determined. Partial rigidification of the calix[4]arene moiety in four different ways was achieved by replacing two of the methoxy groups of the tetramethyl ether 2 by ethoxy groups. The relative thermodynamic stabilities of the conformations of the calix[4]arene are influenced strongly by this relatively small change; in particular the 1,2-alternate conformation becomes much more stable. For the anti-1,3-diethyl-2,4-dimethyl ether 7b the 1,2-alternate is even the thermodynamically most stable conformation. Molecular mechanics calculations indicate that this is caused by the combined favorable effects on the electrostatic energy of the inside orientation of the methoxy groups and the relative large distance between the two ethoxy groups. The tetraethyl ether of p-tert-butylcalix[4]arene is not flexible at room temperature, but it equilibrates in solution at temperatures above 100 "C to a mixture of also all the four possible conformations.

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subsequent dialkylation with NaH/DMF and KOtBuITHF, respectively. In the solid state (X-ray), compound 6 a adopts a flattened cone conformation, which is also found to be most abundant in CD,CN and CD,OD solution. Upon complexation with potassium picrate compound 6a was converted quantita-