129Xe and 1H 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-tetrachloroethane-d(2) is investigated by Xe-129 and H-1 NMR spectroscopy. Xenon is reversibly trapped into the cavity of this host to form a 1 to 1 host-guest complex with an apparent association constant K of the order of at least 3 x 10(3) M-1 at 278 K. The exchange between the free and bound xenon is slow on the Xe-129 NMR time scale, and the bound xenon resonance is shifted by approximately 160 ppm to lower frequencies with respect 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 this xenon complex appears to be much greater than that of the previously described xenon complex of alpha-cyclodextrin in water, This is probably due to the combination of three favorable effects: (i) good size matching between the guest and the cryptophane cavity in its most relaxed conformation, resulting in the optimization of the London forces between the highly polarizable guest and the electron rich aromatic rings of the host (enthalpic stabilization); (ii) no rotational or vibrational entropy loss of the monatomic guest in the cryptophane cavity; and (iii) no (or little) entropy loss due to reduction of the conformational freedom of the host, Analysis of the line widths of the signals corresponding to the free and bound xenon as a function of the relative xenon/cryptophane ratio suggests that the incoming xenon atom must displace the departing one to enter the cryptophane cavity, and that the empty cryptophane is not involved in the complexation equilibrium.