Electron exchange between α-Keggin tungstoaluminates and a well-defined cluster-anion probe for studies in electron transfer

Yurii V. Geletii, Craig L. Hill, Alan J. Bailey, Kenneth I. Hardcastle, Rajai H. Atalla, Ira A. Weinstock

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34 Scopus citations

Abstract

Fully oxidized α-AlIIIW12O40 5- (1ox), and one-electron-reduced α-Al IIIW12O406- (1red), are well-behaved (stable and free of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Having established this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H 2O)63+; ν1/2 = 0.77 Hz) and 1red (74.1 ppm; ν1/2v2 = 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 at the 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 + 2αz 1z2μ1/2/(1 + βrμ1/2), where z1 and z2 are the charges of 1ox and 1red, α and β are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of a Keggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whose slope gives a charge product, z1z2, of 29 ± 2, statistically identical to the theoretical value of 30. Extrapolation to μ = 0 gives a rate constant k 11 of (6.5 ± 1.5) × 10-3 M-1 s-1, more than 7 orders of magnitude smaller than the rate constant [(1.1 ± 0.2) × 105 M-1 s-1] determined by 31P NMR for self-exchange between PVW 12O403- and its one-electron-reduced form, PVW12O404-. Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend 1red as a well-behaved electron donor 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.

Original languageEnglish
Pages (from-to)8955-8966
Number of pages12
JournalInorganic Chemistry
Volume44
Issue number24
DOIs
StatePublished - 28 Nov 2005
Externally publishedYes

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