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| - Quinones as Electron Acceptors. X-Ray Structures, Spectral(EPR, UV−vis) Characteristics and Electron-TransferReactivities of Their Reduced Anion Radicals as Separated vsContact Ion Pairs
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| - Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitablefor X-ray crystallographic analysis allows for the first time their rigorous structural distinction as “separated”ion pairs (SIPs) vs “contact” ion pairs (CIPs). The quantitative evaluation of the precise changes in thegeometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q-•) is viewedrelative to the corresponding (two-electron) reduction to the hydroquinone (H2Q) via the Pauling bond-length/bond-order paradigm. Structural consequences between such separated and contact ion pairs asdefined in the solid state with those extant in solution are explored in the context of their spectral (EPR,UV−vis) properties and isomerization of tightly bound CIPs. Moreover, the SIP/CIP dichotomy is alsoexamined in intermolecular interactions for rapid (self-exchange) electron transfer between Q-• and Q withsecond-order rate constants of kET ≈ 108 M-1 s-1, together with the spectral observation of the paramagneticintermediates [Q,Q-•] leading to 1:1 adducts, as established by X-ray crystallography.
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