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

Abstract

The use of SF6 as a spy molecule in solution-state NMR is investigated as an alternative to 129Xe NMR. 19F chemical shift, longitudinal relaxation time, and integral measurements, as well as intermolecular nuclear Overhauser effects, are reported for SF6 dissolved in simple deuterated solvents and/or in various solutions among which are aqueous solutions of cucurbit[6]uril (CB) and alpha-cyclodextrin (alphaCD). Both CB and alphaCD form a 1:1 inclusion complex with SF6. In a 0.2 M D2SO4-D2O solution, the affinity of CB for SF6 is 2.9x10(5) 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 0.2 M Na2SO4-D2O solution, and the role of the cation is evidenced. In D2O, the affinity of alphaCD for SF6 is about 25 L mol(-1). With CB, the kinetics is slow on both the 1H and 19F chemical shift time scales, while with alphaCD fast exchange is observed. The 19F chemical shift of SF6 is much less sensitive to medium effects than the chemical shift of 129Xe. This might be a limitation if minute structural changes are to be investigated but, in the present study, turned out to be an advantage for determining the affinity of alphaCD for SF6. With CB, the 19F chemical shift of included SF6 is found to be 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 found to be rather sensitive to the environment and was used to determine the affinity of alphaCD 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 concentrations of the order of 10 microM were determined by 19F NMR signal integral measurements and used to quantify the affinity of CB for SF6. Integrals, which are usually not reliable for quantitative measurements in 129Xe NMR, were proven to be highly valuable for determining the affinity of alphaCD 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 alphaCD.