Handbook of Solvents - George Wypych - ChemTech - Ventech!

768 Roland Schmid
from the explicit inverse temperature dependence Ed ∝1/T. Since the liquid is less packed at
higher temperature, less energy is needed for reorganization. Repacking of the solvent
should lead to larger entropy changes than those of dipoles’ reorientation that is enthalpic in
nature due to the long-range character of dipole-dipole forces. The orientational component
increases with temperature essentially as predicted by continuum theories. In these ways the
two solvent modes play complementary roles in the solvent’s total response. This feature
would lead to curved Arrhenius plots of ET rates with slight curvatures in the normal region
of ET (-ΔFo Er). 162,163 The maximum
may however be suppressed by intramolecular reorganization and should therefore be
discovered particularly for rigid donor-acceptor pairs.
Photoinduced ET in binuclear complexes with localized electronic states provides at
the moment the best test of theory predictions for the solvent dependent ET barrier. This
type of reaction is also called metal-metal charge-transfer (MMCT) or intervalence transfer
(IT). The application of the theory to IT energies for valence localized biruthenium complexes
and the acetylene-bridged biferricenium monocation 164 revealed its superiority to
continuum theories. The plots of Es vs. Eop are less scattered, and the slopes of the best-fit
lines are closer to unity. As a major merit, the anomalous behavior of some solvents in the
continuum description - in particular HMPA and occasionally water - becomes resolved in
terms of the extreme sizes, as they appear at the opposite ends of the solvent diameter scale.
Recently, it became feasible for the first time, to measure experimentally for a single
chemical system, viz. a rigid, triply linked mixed-valence binuclear iron polypyridyl complex,
[Fe(440) 3Fe] 5+ , the temperature dependencies of both the rate of thermal ET and the
optical IT energy (in acetonitrile-d3). 165 The net Er associated with the intramolecular electron
exchange in this complex is governed exclusively by low frequency solvent modes,
providing an unprecedented opportunity to compare the parameters of the theories of thermal
and optical ET in the absence of the usual complications and ambiguities. Acceptable
agreement was obtained only if solvent density fluctuations around the reacting system
were taken into account. In these ways the idea of density fluctuations is achieving experimental
support. The two latest reports on negative temperature coefficients of the solvent
reorganization energy (decrease in Es with temperature) should also be mentioned. 166,167
Thus, two physically important properties of molecular liquids are absent in the continuum
picture: the finite size of the solvent molecules and thermal translational modes resulting
in density fluctuations. Although the limitations of the continuum model are long
known, the necessity for a molecular description of the solvent, curiously, was first recognized
in connection with solvent dynamic effects in ET. Solvent dynamics, however, affects
the preexponent of the ET rate constant and, therefore, influences the reaction rate much
less than does the activation energy. From this viewpoint it is suspicious that the ET activation
energy has so long been treated in the framework of continuum theories. The reason of
this affection is the otherwise relative success of the latter, traceable to two main features.
First, the solvents usually used are similar in molecular size. Second, there is a compensation
because altering the size affects the orientational and translational parts of the solvent
barrier in opposite directions.
The solution ionic radius
The solution ionic radius is arguably one of the most important microscopic parameters. Although
detailed atomic models are needed for a full understanding of solvation, simpler
phenomenological models are useful to interpret the results for more complex systems. The