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

Wheate, N.J. and Kumar, P.G.A. and Torres, A.M. and Aldrich-Wright, J.R. and Price, W.S. (2008) Examination of cucurbit[7]uril and its host-guest complexes by diffusion nuclear magnetic resonance. Journal of Physical Chemistry B, 112 (8). pp. 2311-2314. ISSN 1520-6106 (http://dx.doi.org/10.1021/jp709847p)

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Abstract

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 m2 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 D2O. 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].

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