Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 753
is substantial for both solvent classes. 56 Recently, this latter explanation in terms of dipole-quadrupole
interactions is favored (Fig 13.1.8).
It is well-known that the interaction energy falls off more rapidly the higher the order
of the multipole. Thus, for the interaction of an n-pole with an m-pole, the potential energy
varies with distance as E ∝ 1/(r n+m+1 ). The reason for the faster decrease is that the array of
charges seems to blend into neutrality more rapidly with distance the higher the number of
individual charges contributing to the multipole. Consequently, quadrupolar forces die off
faster than dipolar forces.
It has been calculated that small solute dipoles are even more effectively solvated by
solvent quadrupoles than by solvent dipoles. 57 In these terms it is understandable that
quadrupolar contributions are more important in the π* than in the ET(30) scale. Similarly,
triethylphosphine oxide, the probe solute of the acceptor number scale, is much smaller than
betaine(30) and thus might be more sensitive to quadrupolar solvation. Thus, at long last,
the shape of Figure 13.1.2 and similar ones seems rationalized. Note by the way that the
quadrupole and CT mechanisms reflect, respectively, inertial and inertialess solvation pathways,
and hence could be distinguished by a comparative analysis of absorption and fluorescence
shifts (Stokes shift analysis). However, for 4-nitroanisole fluorescence data are not
available.
Reverting once more to the thermodynamic analysis of the π* and ET(30) scales referred
to above, it should be mentioned that there are also other theoretical treatments of the
solvatochromism of betaine(30). Actually, in a very recent computer simulation, 58 the large
polarizability change Δα (nearly 2-fold, see Table 13.1.4) upon the excitation of betaine(30)
has been (correctly) questioned. (According to a rule of thumb, the increase in polarizability
upon excitation is proportional to the ground state polarizability, on the order
Δα ≈ 0.25α g .50 ) Unfortunately, Matyushov et al. 55 derived this high value of Δα =61Å 3
from an analysis of experimental absorption energies based on aromatic, instead of alkane,
solvents as nonpolar reference solvents. A lower value of Δα would diminish the importance
of dispersion interactions.
Further theoretical and computational studies of betaine(30) of the ET(30) scale are reviewed
by Mente and Maroncelli. 58 Despite several differences in opinion obvious in these
papers, an adequate treatment of at least the nonspecific components of solvatochromism
would seem to be “just around the corner”. Finally, a suggestion should be mentioned on
using the calculated π* values taken from ref. 55 as a descriptor of nonspecific solvent effects.
59 However, this is not meaningful since these values are just a particular blend of inductive,
dispersive, and dipole-dipole forces.
13.1.7 SOME HIGHLIGHTS OF RECENT INVESTIGATIONS
The like dissolves like rule
The buzzword “polarity”, derived from the dielectric approach, is certainly the most popular
word dealing with solvent effects. It is the basis for the famous rule of thumb “similia
similibus solvuntur” (“like dissolves like”) applied for discussing solubility and miscibility.
Unfortunately, this rule has many exceptions. For instance, methanol and toluene, with dielectric
constants of 32.6 and 2.4, respectively, are miscible, as are water (78.4) and
isopropanol (18.3). The problem lies in exactly what is meant by a “like” solvent. Originally,
the term “polarity” was meant to be an abbreviation of “static dipolarity” and was
thus associated with solely the dielectric properties of the solvent. Later on, with the advent