Diffusion coefficient of cucurbit[n]urils (n=6 or 7) at various concentrations, temperatures, and pH

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 H2O than D2O with increasing temperature, consistent with the Stokes−Einstein equation where diffusion is proportional to temperature divided by viscosity. The activation energy (EA) was also determined and is larger for CB[n] in 90 % H2O/10 % D2O v/v than in D2O. 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.