XE-129 AND H-1-NMR STUDY OF THE REVERSIBLE TRAPPING OF XENON BY CRYPTOPHANE-A IN ORGANIC SOLUTION

The interaction of xenon with cryptophane-A in I,1,2,2-tetrachloroetha ne-d(2) is investigated by Xe-129 and H-1 NMR spectroscopy. Xenon is r eversibly trapped into the cavity of this host to form a 1 to 1 host-g uest complex with an apparent association constant K of the order of a t least 3 x 10(3) M-1 at 278 K. The exchange between the free and boun d xenon is slow on the Xe-129 NMR time scale, and the bound xenon reso nance is shifted by approximately 160 ppm to lower frequencies with re spect to the free xenon resonance, The xenon complex is at least 4 and 20 times more stable, respectively, than the corresponding chloroform and methane complexes under the same conditions. The stability of thi s xenon complex appears to be much greater than that of the previously described xenon complex of alpha-cyclodextrin in water, This is proba bly due to the combination of three favorable effects: (i) good size m atching between the guest and the cryptophane cavity in its most relax ed conformation, resulting in the optimization of the London forces be tween the highly polarizable guest and the electron rich aromatic ring s of the host (enthalpic stabilization); (ii) no rotational or vibrati onal entropy loss of the monatomic guest in the cryptophane cavity; an d (iii) no (or little) entropy loss due to reduction of the conformati onal freedom of the host, Analysis of the line widths of the signals c orresponding to the free and bound xenon as a function of the relative xenon/cryptophane ratio suggests that the incoming xenon atom must di splace the departing one to enter the cryptophane cavity, and that the empty cryptophane is not involved in the complexation equilibrium.