Diffusion Coefficient of Cucurbit[<i>n</i>]urils (<i>n</i> = 6 or 7) at Various Concentrations, Temperatures, and pH

Abstract

The diffusion coefficient of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) was determined using pulsed gradient spin−echo nuclear magnetic resonance spectroscopy. Both CB[6] and CB[7] diffuse faster through H 2 O than D 2 O with increasing temperature, consistent with the Stokes−Einstein equation where diffusion is proportional to temperature divided by viscosity. The activation energy ( E A ) was also determined and is larger for CB[ n ] in 90 % H 2 O/10 % D 2 O v/v than in D 2 O. Both compounds aggregate with increasing concentration between (0.25 and 2)·10 −3 mol·L −1 . The pH of the solution does not significantly affect the diffusion coefficient of CB[6] and CB[7], except at very low pH where protonation of the carbonyl groups induces aggregation, which slows their rate of diffusion.