Conformational interconversion bridged calix arenes

The conformational mobility of the bridged calix[6]arenes of type 11 is largely dependent on the capping groups of the lower rim. In solution, compound 13 bearing four methyl caps in the lower rim was found to undergo slow conformational interconversion on the NMR timescale at room temperature, although it adopted the 1,2,3-alternate conformation in the crystalline state. °... 

The parent calix[4]arenes are (exclusively) found in the so-called cone conformation, where all the OH groups point in one direction which is therefore stabilized by an intramolecular array of hydrogen bonds. From variable temperature NMR studies (showing at low temperature a pair of doublets and at high temperature a singlet for the protons of the methylene bridges) the energy barrier. AG for the interconversion between two... 

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