Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 585
ever since Lewis unified the acidity and basicity concepts in 1923, 19 it has been a constant
challenge for chemists to find a single quantifiable property of solvents that could serve as a
general basicity indicator. Specially significant among the attempts at finding one are the
donor number (DN) of Gutmann et al., 20 the B(MeOD) parameter of Koppel and Palm, 21 the
pure base calorimetric data of Arnet et al. 22 and parameter β of Kamlet and Taft. 18b
10.3.2 CHARACTERIZATION OF A MOLECULAR ENVIRONMENT WITH THE
AID OF THE PROBE/HOMOMORPH MODEL
As a rule, a good solvent probe must possess two energy states such that the energy or intensity
of the transition between them will be highly sensitive to the nature of the environment.
The transition concerned must thus take place between two states affected in a different
manner by the molecular environment within the measurement time scale so that the transition
will be strongly modified by a change in the nature of the solvent. For easier quantification,
the transition should not overlap with any others of the probe, nor should its spectral
profile change over the solvent range of interest. Special care should also be exercised so
that the probe chosen will be subject to no structural changes dependent on the nature of the
solvent; otherwise, the transition will include this perturbation, which is external to the pure
solvent effect to be quantified.
All probes that meet the previous requirements are not necessarily good solvent
probes, however; in fact, the sensitivity of the probe may be the result of various types of interaction
with the solvent and the results difficult to generalize as a consequence. A probe
suitable for determining a solvent effect such as polarity, acidity or basicity should be
highly sensitive to the interaction concerned but scarcely responsive to other interactions so
that any unwanted contributions will be negligible. However, constructing a pure solvent
scale also entails offsetting these side effects, which, as shown later on, raises the need to
use a homomorph of the probe.
The use of a spectroscopic technique (specifically, UV-Vis absorption
spectrophotometry) to quantify the solvent effect provides doubtless advantages. Thus, the
solute is in its electronic ground state, in thermodynamic equilibrium with its environment,
so the transition is vertical and the solvation sphere remains unchanged throughout. These
advantages make designing a good environmental probe for any of the previous three effects
quite easy.
A suitable probe for the general solvent effect must therefore be a polar compound.
Around its dipole moment, the solvent molecules will arrange themselves as effectively as
possible -in thermal equilibrium- and the interaction between the probe dipole and the solvent
molecules that form the cybotactic region must be strongly altered by electronic excitation,
which will result in an appropriate shift in charge; the charge will then create a new
dipole moment enclosed by the same cybotactic cavity as in the initial state of the transition.
The change in the dipole moment of the probe will cause a shift in the electronic transition
and reflect the sensitivity of the probe to the solvent polarity and polarizability. A high
sensitivity in the solvent is thus usually associated with a large change in the dipole moment
by effect of the electronic transition. However, the magnitude of the spectral shift depends
not only on the modulus of the dipole moment but also on the potential orientation change.
In fact, inappropriate orientation changes are among the sources of contamination of polarity
probes with specific effects. For simplicity, orientation changes can be reduced to the
following: (a) the dipole moment for the excited state is at a small angle to that for the
ground state of the probe, so the orientation change induced by the electronic excitation can