Does Marcus-Hush theory really work? Optical studies of intervalence transfer in acetylene-bridged biferrocene monocation at infinite dilution and at finite ionic strengths
The Journal of Physical Chemistry
Intervalence charge-transfer data for acetylene-bridged biferrocene monocation (Sf+) have been collected in five solvents in the presence and absence of excess electrolyte and in the limit of infinite chromophore dilution. The study was motivated by earlier work which demonstrated that the intervalence absorption maximum for BfY in methylene chloride could vary substantially with both chromophore concentration and added electrolyte concentration. In the present study similar (but smaller)
... (but smaller) variations are found in other solvents. The variations are ascribable to both ion pairing and higher order ionic association. For the available solvents the logarithms of the BfYX-pairing constant (X-= PF;, BF4-, or C104-) is found to vary inversely with the solvent's static dielectric constant, 0,. When the ion-pairing effects are eliminated by dilution, a genuine test of the Marcus-Hush prediction of the solvent dependence of the intervalence charge-transfer energy (EgMCT) is possible. For highly polar solvents (D, 2 20) a good fit of EgMCT to l/Do,l / D s is found (where Dop is the optical dielectric constant and I/DOp is the main variant in the two-term dielectric parameter). For methylene chloride, however, ,?EMcT falls well below the best-fit line for five other solvents. The deviation is tentatively attributed to dielectric saturation and is expected, therefore, to be manifest in other media possessing limited dielectric strength. In light of these findin s a speculative interpretation of pressure-induced solvent freezing effects is offered. It is suggested that, in frozen CD,CN, EgM'i s simultaneously subjected to ion-pairing perturbations and to dielectric saturation effects and that the effects act in an offsetting, largely compensatory fashion. The speculation is supported by semiquantitative extrapolations from known liquid solvent behavior and by an analysis of intervalence bandwidths. ( I ) Historical reviews: (a) Marcus, R. The molecular structure and conformation of diisopropyl ketone, MezC(3)HC(2)OC(4)HMez, at 24 'C have been investigated by gas electron diffraction with the aid of vibrational spectroscopy and ab initio SCF calculations at the 4-21G level. Three conformers with CI, C" and C2 symmetry exist with the molar fractions of 0.45 (31), 0.31 (12), and 0.24 (22), respectively. The dihedral angles $I(C4CzC3H) and $2(C3C2C4H) of the C" C" and C2 conformers (the values are denoted by $z)) are (16', -62'), (OO, 180°), and (59', 59'), respectively, where 4, and $z are defined to be zero when the C-C bond eclipses the C-H bond. The main structural parameters (r and La) of the CI conformer with the limits of error (3a) in parentheses are as follows: r(C=O) = 1.215 (5) A, (r(C-Cf) = 1.535 (2) A, r(C-H) = 1.118 (3) A, LCC(=O)C = 116.6 (17)', (LCCC) = 110.8 (4)O, and LCCHM, = 11 1.1 (9)', where the symbol ( ) denotes average values.