An NMR Study on Complexation of Cesium Ion by p-tert-Butylcalix[6]arene Ethyl Ester

Complexation of cesium ion by p-tert-butylcalix[6]arene ethyl ester was studied by NMR spectroscopy in nonpolar CDCl₃ and polar acetone-d₆ and the results were compared with each other. Analysis of temperature dependent ¹H spectra and titration curves reveals that both solvents result in a 1 : 1 cone-form complex with nonpolar CDCl₃ yielding a more tightly bound one than acetone-d₆. Unexpectedly, at very low temperature, we have found that two phenyl ring proton peaks of equal intensity appear both in CDCl₃ and in acetone-d₆ solution which gradually collapse and eventually coalesce into a single line as temperature is raised. This observation could be interpreted in terms of the chemical exchange through direct and/or indirect interconversion between two equivalent conformations possible the complex in both solvents over the temperature range observed. And broadening of ¹³³Cs (I = 7/2) nmr line with increasing temperature has also been observed, indicating the exchange of ¹³³Cs ion between the complex and the solvent. From numerical fitting of lineshape changes for one-dimensional ¹H and ¹³³Cs spectra, the exchange rate constants and other relevant parameters for this conformational interconversion and the complex-solvent exchange were deduced