Handbook of Solvents - George Wypych - ChemTech - Ventech!

608 Javier Catalán
Figure 10.3.5. Plot of Allerhand and Schleyer’s G values
vs. the predicted G values according to eq.
[10.3.20].
Figure 10.3.6. Plot of Reichardt’s E T(30) values vs. the
predicted E T(30) values according to eq. [10.3.21].
For the 138 solvents with a known E T(30) value listed in Table 10.3.1, this parameter
reflects mostly solvent acidity and polarity, and, to a lesser degree, also solvent basicity (see
Figure 10.3.6):
E T(30) = 0.62(±0.04)SPP + 0.12(±0.02)SB + 0.77(±0.02)SA -0.31(±0.03) [10.3.21]
with r = 0.965 and sd = 0.06.
It should be noted that the three solvent effects (polarity, acidity and basicity) contribute
to increasing ET(30), i.e., to a hypsochromic shift in the transition of the probe (see Figure
10.3.6).
The spectroscopic behavior of the probe used to construct this scale (1) is typical of a
structure exhibiting highly localized charge on its carbonyl group in the electronic ground
state, see scheme III, and hence strong stabilization by effect of increased solvent polarity
and acidity. The electronic transition delocalizes the charge and results in an excited state
that is much less markedly stabilized by increased polarity or acidity in the solvent. The decreased
dipole moment associated to the electronic transition in this probe also contributes
to the hypsochromic shift. The small contribution of solvent basicity to the transition of the
probe (1) is not so clear, however.
10.3.7.2 Treatment of the solvent effect in:
10.3.7.2.1 Spectroscopy
Analyzing the SPP, SB and SA scales in the light of spectroscopic data that are sensitive to
the nature of the solvent poses no special problem thanks to the vertical nature of the transitions,
where the cybotactic region surrounding the chromophore is hardly altered. The papers
where the SPP, SA and SB scales were reported discuss large sets of spectroscopic data
in terms of the nature of the solvent. Some additional comments are made below.
Recently, Fawcet and Kloss, 83 analyzed the S=O stretching frequencies of dimethyl
sulfoxide (DMSO) with a view to elucidating the behavior of 20 solvents and the gas phase.