Handbook of Solvents - George Wypych - ChemTech - Ventech!

744 Roland Schmid
ated by the macroscopically attainable fields. A correlation similar to [13.1.5] has been
proposed 20 between the gas phase dipole moment and π*
()
μ D = 43 . π*
−01
. [13.1.6]
n = 28, r = 0.972, s = 0.3
Along these lines the dielectric and the chemical approach are brought under one
roof. 4,21 The statement, however, that the terms “good acceptor solvent” and “highly polar
solvent” may be used synonymously would seem, though true, to be provocative.
13.1.5 CONCEPTUAL PROBLEMS WITH EMPIRICAL SOLVENT
PARAMETERS
A highly suspect feature behind the concept of empirical solvent parameters lies in the interpretation
of the results in that condensed phase matters are considered from the narrow
viewpoint of the solute only with the solvent’s viewpoint notoriously neglected. However,
the solute is actually probing the overall action of the solvent, comprising two modes of interactions:
solute-solvent (solvation) and solvent-solvent (restructuring) effects of unknown
relative contribution. Traditionally, it is held that solvent structure only assumes
importance when highly structured solvents, such as water, are involved. 22 But this view increasingly
turns out to be erroneous. In fact, ignoring solvent-solvent effects, even in
aprotic solvents, can lead to wrong conclusions as follows.
In the donor-acceptor approach, solutes and solvents are divided into donors and acceptors.
Accordingly, correlations found between some property and the solvent donor (acceptor)
ability are commonly thought to indicate that positive (negative) charge is involved.
In the case of solvent donor effects this statement is actually valid. We are unaware, in fact,
of any exception to the rule saying: “Increase in reaction rate with increasing solvent DN
implies that positive charge is developed or localized and vice versa”. 21 In contrast, correlations
with the acceptor number or related scales do not simply point to anion solvation,
though this view is commonly held. An example for such type of reasoning concerns the
medium effect on the intervalence transition (IT) energy within a certain binuclear,
mixed-valence, 5+ cation. 23 As the salt effect was found to vary with the solvent AN, anion,
that is counterion, solvation in ion pairs was invoked to control the IT energy.
A conceptual problem becomes obvious by the at first glance astonishing result that
the reduction entropies of essentially non-donor cationic redox couples such as
3+/2+ 24
Ru(NH3) 6 are correlated with the solvent AN. These authors interpreted this solvent
dependence as reflecting changes in solvent-solvent rather than solvent-ligand interactions.
That the acceptor number might be related to solvent structure is easy to understand since all
solvents of high AN always are good donors (but not vice versa!) and therefore tend to be increasingly
self-associated. 21 There is since growing evidence that the solvent’s AN and related
scales represent ambiguous solvent properties including solvent structural effects
instead of measuring anion solvation in an isolated manner. Thus, correlations between
Gibbs energies of cation transfer from water to organic solvents and the solvent DN are improved
by the inclusion of a term in ET (or a combination of α and π*). 25 Consequently
Marcus et al. rightly recognized that “ET does not account exclusively for the electron pair
acceptance capacity of solvents”. 26 In more recent work 27 a direct relationship has been