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

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.