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-d(6) and the results were compared with each other. Analysis of temperature dependent H-1 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-d(6). Unexpectedly, at very low temperature, we have found that two phenyl ring proton peaks of equal intensity appear both in CDCl3 and in acetone-d(6) 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 Cs-133 (I = 7/2) nmr line with increasing temperature has also been observed, indicating the exchange Of Cs-133 ion between the complex and the solvent. From numerical fitting of lineshape changes for one-dimensional H-1 and Cs-133 spectra, the exchange rate constants and other relevant parameters for this conformational interconversion and the complex-solvent exchange were deduced.