Solvent Effects on Intervalence Electron-Transfer Energies for Biferrocene Cations: Comparisons with Molecular Models of Solvent Reorganization

摘要

Outer-shell electron-transfer reorganization energies, lambda sub O, as evaluated from intervalence band maxima, E sub op, for biferrocenylacetylene (BFA+), 1,4-biferrocenlbutadiyne (BFB+), and biferrocene (BF+) cations in thirteen solvents are compared with the predictions of some contemporary models of solvent reorganization in order to ascertain the importance of 'non-continuum', or solvent molecularity, factors to lambda sub O. Measurements of E sub op were made in relatively polar media in solutions containing low concentrations of PF6(-) so to minimize complications from ion pairing. The relative E sub OP values for all three biferrocenes are essentially independent of the solvent; E sub op for BFB+ relative to BFA+ is in good agreement with the predictions of the two-sphere model (but not ellipsoid models), although the Eop values for BF+ are larger than expected, most probably due to electron delocalization effects. Comparisons are made with the recent mean spherical approximation (MSA) treatment of solvent reorganization. The MSA model can account semiquantitatively for the smaller E sub op values relative to E sub op (con) observed in a number of solvents, but does not predict the especially large such deviations observed for methanol and D20. The latter are shown to be qualitatively consistent with local 'solvent structuring' effects deduced on the basis of a nonlocal dielectric approach as well as on empirical grounds. (jhd)