Examination of cucurbit[7]uril and its host-guest complexes by diffusion nuclear magnetic resonance

The self-diffusion of cucurbit[7]uril (CB[7]) and its host-guest complexes in D₂O has been examined using pulsed gradient spin-echo nuclear magnetic resonance spectroscopy. CB[7] diffuses freely at a concentration of 2 mM with a diffusion coefficient (D) of 3.07 × 10^-10 m ² s^-1. At saturation (3.7 mM), CB[7] diffuses more slowly (D = 2.82 × 10^-10 m² s^-1) indicating that it partially self-associates. At concentrations between 2 and 200 mM, CsCl has no effect on the diffusion coefficient of CB[7] (1 mM). Conversely, CB[7] (2 mM) significantly affects the diffusion of ¹³³Cs⁺ (1 mM), decreasing its diffusion coefficient from 1.86 to 0.83 × 10^-9 m² s^-1. Similar changes in the rate of diffusion of other alkali earth metal cations are observed upon the addition of CB[7]. The diffusion coefficient of ²³Na⁺ changes from 1.26 to 0.90 × 10^-9 m² s^-1 and ⁷Li ⁺ changes from 3.40 to 3.07 × 10^-9 m² s^-1. In most cases, encapsulation of a variety of inorganic and organic guests within CB[7] decreases their rates of diffusion in D ₂O. For instance, the diffusion coefficient of the dinuclear platinum complex trans-[{PtCl(NH₃)₂}₂μ-dpzm] ²⁺ (where dpzm is 4,4'-dipyrazolylmethane) decreases from 4.88 to 2.95 × 10^-10 m² s^-1 upon encapsulation with an equimolar concentration of CB[7].