NMR Study of the Reversible Trapping of SF6 by Cucurbit[6]uril in Aqueous Solution.

The complexation of SF6, a highly potent greenhouse gas, by cucurbit[6]uril (CB) was studied at various temps. in Na2SO4 aq. solns. by 19F and 1H NMR. CB shows a remarkable affinity for SF6, suggesting that it is a suitable mol. container for the design of materials tailored for SF6 trapping. At 298 K, the equil. const. characterizing the inclusion of SF6 by CB is 3.1 × 104/M and the residence time of SF6 within the CB cavity is estd. to be of the order of a few seconds. The enthalpic and entropic contributions to the free energy of encapsulation were detd. and are discussed. This work also reports on the interest of SF6 in the framework of the spin-spy methodol. The advantages and drawbacks of soln.-state 19F NMR of SF6 with respect to 129Xe NMR are discussed. SF6 comes forward as a versatile and informative spin-spy mol. for probing systems in soln. because its detection limit by 19F NMR reaches the micromolar range with std. equipment and because quant. integral measurements, relaxation time measurements, and demanding expts., such as translational diffusion coeff. measurements, are easily carried out in addn. to chem. shift measurements. Soln.-state 19F NMR of SF6 emerges as a promising alternative to 129Xe NMR for probing cavities and for other applications relying on the encapsulation of a NMR active gaseous probe. [on SciFinder(R)]