title: Isolation of the latent precursor complex in electron-transfer dynamics. Intermolecular association and self-exchange with acceptor anion radicals.
abstract: Transient [1:1] complexes formed in the bimolecular interactions of electron acceptors (A) with their reduced anion radicals (A(-.)) are detected and characterized in solution for the first time. The recognition of such metastable intermediates as the heretofore elusive precursor complex (A(2)(-.)) in electron-transfer processes for self-exchange allows the principal parameters lambda (Marcus reorganization energy) and H(DA) (electronic coupling element) to be experimentally determined from the optical (charge-transfer) transitions inherent to these intermolecular complexes. The satisfactory correspondence of the theoretically predicted with the experimentally observed rate constants validates these ET parameters and the Marcus-Hush-Sutin methodology for strongly coupled redox systems lying in the (Robin-Day) Class II category. Most importantly, the marked intermolecular electronic interaction (H(DA)) within these precursor complexes must be explicitly recognized, since it dramatically affects the electron-transfer dynamics by effectively lowering the activation barrier. As such, the numerous calculations of the reorganization energy previously obtained from various self-exchange kinetics based on lambda = 4DeltaG must be reconsidered in the light of such a precursor complex, with the important result that ET rates can be substantially faster than otherwise predicted. On the basis of these studies, a new mechanistic criterion is proposed for various outer-sphere/inner-sphere ET processes based on the relative magnitudes of H(DA) and lambda.
authors.affiliation: Department of Chemistry, University of Houston, Houston, TX 77204, USA.
authors.forename: Vellaichamy
authors.initials: V
authors.lastname: Ganesan
authors.forename: Sergiy V
authors.initials: SV
authors.lastname: Rosokha
authors.forename: Jay K
authors.initials: JK
authors.lastname: Kochi