NMR study of the reversible complexation of xenon by cucurbituril

The interaction of xenon with cucurbituril, a synthetic receptor, in aqueous solutions is investigated by Xe-129 and H-1 NMR spectroscopy. Xenon is reversibly trapped into the cavity to form a 1:1 host-guest complex. The exchange between the free and the complexed xenon is slow on the Xe-129 NMR chemical shift time scale but fast on the H-1 NMR chemical shift time scale. The apparent association constant of xenon for cucurbituril can be obtained from analysis of the H-1 spectra and is estimated to be around 200 M-1 at 298 K. Interestingly, even though no H-bonding or electrostatic interactions play a role in the stabilisation of the complex, its stability is comparable to the stability of cucurbituril complexes with certain aryl- and alkyl-ammonium ions.