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Effective distances of exothermic charge-transfer reactions in the excited state

Costa, Sílvia M. B.; Maçanita, A. L.; Formosinho, S. J.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
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A study of intermolecular, exothermic charge-transfer interactions is described. A distance dependence on the unimolecular charge-transfer rate constant k,,(R) is observed in viscous solvents where transfer at longer distances than the collisional one can compete with dissociation of an encounter complex. A comparative study of the quenching phenomena, occurring in a family of redox partners of aromatic esters and amines, was carried out in nonviscous solvents (hexane and acetonitrile) and viscous solvents (liquid paraffin/heptane mixtures and nonionic micelles of Triton X- 100). In the latter, the experimental reactional distances obtained are discussed in terms of effective distances R,ff, which are related to an average value of kct(Reff) for the reactant pair distribution function. A correlation of these distances with either the free activation energy, Act, or the diffusion coefficient D enables the estimation of an order of magnitude of parameters such as the average radii of donor and acceptor orbitals, L = (2.7 f 0.2) X cm, a preexponential factor at the collisional distance, k’,,(R,) = 3.7 X 10” d, an intrinsic reorganization parameter X = 0.64 eV, and an electronic matrix element V(Rc) N eV for the nonpolar media studied. An equation for kct(R)...