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The host–guest stability constants for the inclusion of a series of small neutral polar organic guests in cucurbit[7]uril (CB[7]) have been determined in aqueous solution by 1H NMR titrations. The dependence of the stability constant on the nature of the guests indicates that hydrophobic and dipole–quadrupole interactions are responsible for the binding. The complexation-induced chemical shift changes in the guest proton resonances, coupled with energy-minimization calculations, suggest that the guests are located such that their dipole moment is aligned perpendicular with the quadrupole moment of the CB[7] host. The stability constants for acetone and acetophenone decrease in the presence of Na+ or K+ cations as a result of cation capping of the CB[7] portals.

Graphical abstract: Cucurbit[7]uril host–guest complexes with small polar organic guests in aqueous solution

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