| Abstract
| - Fully oxidized α-AlIIIW12O405- (1ox), and one-electron-reduced α-AlIIIW12O406- (1red), are well-behaved (stable andfree of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Havingestablished this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H2O)63+; ν1/2 = 0.77 Hz) and 1red (74.1 ppm; ν1/2 = 0.76 Hz). Bimolecular rate constants,k, are obtained from line broadening in 27Al NMR signals as ionic strength, μ, is increased by addition of NaCl atthe slow-exchange limit of the NMR time scale. The dependence of k on μ is plotted using the extended Debye−Hückel equation: log k = log k0 + 2αz1z2μ1/2/(1 + βrμ1/2), where z1 and z2 are the charges of 1ox and 1red, α andβ are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of aKeggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whoseslope gives a charge product, z1z2, of 29 ± 2, statistically identical to the theoretical value of 30. Extrapolation toμ = 0 gives a rate constant k11 of (6.5 ± 1.5) × 10-3 M-1 s-1, more than 7 orders of magnitude smaller than therate constant [(1.1 ± 0.2) × 105 M-1 s-1] determined by 31P NMR for self-exchange between PVW12O403- and itsone-electron-reduced form, PVW12O404-. Sutin's semiclassical model reveals that this dramatic difference arisesfrom the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend1red as a well-behaved electron donor for investigating outer-sphere electron transfer to molecules or nanostructuresin water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres,for which 1ox and 1red provide a working model.
- Line broadening in 27Al NMR spectra is used to measure the rate of electron self-exchange between fully oxidized α-AlIIIW12O405- (1ox) and one-electron-reduced α-AlIIIW12O406- (1red), at neutral pH in water. These data recommend 1red as a well-behaved and kinetically well-defined “probe” for investigating outer-sphere electron transfer to molecules or nanostructures in water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres, for which 1ox and 1red provide a working model.
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