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 H ₂0 than D₂0 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 % H₂O/10 % D₂0 v/v than in D₂0. 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.