NMR investigation of the complexation of neutral guests by cucurbituril

The effect of acidity on the stability of the complexes of neutral guests w ith cucurbituril (CB), a synthetic receptor, in aqueous Na2SO4 solutions is investigated by Xe-129, F-19 and H-1 NMR spectroscopy. The apparent affini ty constants of THF and xenon for CB were measured and found to be independ ent of the pH of the solution. It was also observed that small acids, such as TFA, in their undissociated form can be encapsulated inside the CB cavit y. The apparent association constants of THF, Xe and TFA for CB are estimat ed to be around 1700 M-1 210 M-1 and 11 M-1. respectively.