Probing Systems in Solution by NMR Using Sulfur Hexafluoride as a Spy Molecule.

The use of SF6 as a spy mol. in soln.-state NMR was studied as an alternative to 129Xe NMR 19F chem. shift, longitudinal relaxation time, and integral measurements, as well as intermol. nuclear Overhauser effects, are reported for SF6 dissolved in simple deuterated solvents and/or in various solns. among which are aq. solns. of cucurbit[6]uril (CB) and α-cyclodextrin (αCD). Both CB and αCD form a 1:1 inclusion complex with SF6. In a 0.2M D2SO4-D2O soln., the affinity of CB for SF6 is 2.9 × 105 L/mol-1 at 298 K; it is the largest value ever found for the inclusion of a neutral guest into the CB cavity. It is one order of magnitude larger than in a 0.2M Na2SO4-D2O soln., and the role of the cation is evidenced. In D2O, the affinity of αCD for SF6 is ∼25 L/mol-1. With CB, the kinetics is slow on both the 1H and 19F chem. shift time scales, while with αCD fast exchange is obsd. The 19F chem. shift of SF6 is much less sensitive to medium effects than the chem. shift of 129Xe. This might be a limitation if minute structural changes are to be studied but, turned out to be an advantage for detg. the affinity of αCD for SF6. With CB, the 19F chem. shift of included SF6 is sensitive to the nature of the cation bound at the portals. The 19F relaxation time of SF6 dissolved in deuterated solvents is dominated by the spin-rotation mechanism and is orders of magnitude shorter that the 129Xe relaxation time. It is rather sensitive to the environment and was used to det. the affinity of αCD for SF6. The detection limit of SF6 by 19F NMR is lower than the NMR detection limit of thermally polarized 129Xe by more than three orders of magnitude. It lies in the micromolar range, and SF6 concns. of the order of 10 μM were detd. by 19F NMR signal integral measurements and used to quantify the affinity of CB for SF6. Integrals, which are usually not reliable for quant. measurements in 129Xe NMR, were proven to be highly valuable for detg. the affinity of αCD for SF6 as well. Most interestingly, heteronuclear Overhauser effect measurements allow to selectively highlight the 1H of a SF6 binding site according to the 19F-1H proximity and were used to characterize the inclusion complex formed with αCD. [on SciFinder(R)]