## Abstract

Fully oxidized α-Al^{III}W_{12}O_{40} ^{5-} (1_{ox}), and one-electron-reduced α-Al ^{III}W_{12}O_{40}^{6-} (1_{red}), 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, ^{27}Al NMR spectroscopy is used to measure rates of electron exchange between 1_{ox} (^{27}Al NMR: 72.2 ppm relative to Al(H _{2}O)_{6}^{3+}; ν_{1/2} = 0.77 Hz) and 1_{red} (74.1 ppm; ν_{1/2}v2 = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in ^{27}Al 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 k_{0} + 2αz _{1}z_{2}μ^{1/2}/(1 + βrμ^{1/2}), where z_{1} and z_{2} are the charges of 1_{ox} and 1_{red}, 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 (R^{2} = 0.996), whose slope gives a charge product, z_{1}z_{2}, 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) × 10^{5} M^{-1} s^{-1}] determined by ^{31}P NMR for self-exchange between P^{V}W _{12}O_{40}^{3-} and its one-electron-reduced form, P^{V}W_{12}O_{40}^{4-}. Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1_{ox} and 1_{red}. These results, including independent verification of k_{11}, recommend 1_{red} 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 1_{ox} and 1_{red} provide a working model.

Original language | English |
---|---|

Pages (from-to) | 8955-8966 |

Number of pages | 12 |

Journal | Inorganic Chemistry |

Volume | 44 |

Issue number | 24 |

DOIs | |

State | Published - 28 Nov 2005 |

Externally published | Yes |