Quinones as Electron Acceptors. X-Ray Structures, Spectral (EPR, UV−vis) Characteristics and Electron-Transfer Reactivities of Their Reduced Anion Radicals as Separated vs Contact Ion Pairs

Successful isolation of a series of pure (crystalline) salts of labile quinone anion radicals suitable
for 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 the
geometries of these quinones (Q) upon one-electron reduction to afford the anion radical (Q-•) is viewed
relative 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 as
defined 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 also
examined in intermolecular interactions for rapid (self-exchange) electron transfer between Q-• and Q with
second-order rate constants of kET ≈ 108 M-1 s-1, together with the spectral observation of the paramagnetic
intermediates [Q,Q-•] leading to 1:1 adducts, as established by X-ray crystallography.