Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 607
Figure 10.3.3. Plot of Dong and Winnck’s P y values vs.
the predicted P y values according to eq. [10.3.18] (Point
for gas phase, �, is included in the plot, but not in the
regression equation [10.3.18]).
Figure 10.3.4. Plot of Dubois and Bienvenüe’s Φ values
vs. the predicted Φ values according to eq.[10.3.19].
precise manner- solvent polarity, as is clearly apparent from eq. [10.3.18] (see Figure
10.3.3):
P y = 2.30(±0.20)SPP - 0.57(±0.16) [10.3.18]
with r = 0.839 and sd = 0.20. The difficulty of understanding the performance of the probe
precludes rationalizing its spectral behavior.
The 19 Φ values examined indicate that, based on eq. [10.3.19] and Figure 10.3.4, this
parameter reflects mostly solvent acidity and, to a lesser extent, solvent polarity:
Φ = 0.18(±0.11)SPP + 0.38(±0.04)SA - 0.07(±0.08) [10.3.19]
with r = 0.941 and sd = 0.05.
The Φ scale is based on the solvatochromic behavior of an n →π* electronic transition,
so it reflects a high hypsochromic sensitivity to solvent acidity (specific solvation between
the lone pair involved in the electronic transition and solvent acidity is lost when one
electron in the pair is electronically excited). An n →π* electronic transition is also known
to decrease the polarity of the chromophore concerned, 82 so an increase in SPP for the solvent
will cause a hypsochromic shift in the electronic transition.
Based on Figure 10.3.5 and the following equation, the values of G parameters are also
dictated by solvent polarity and, to a lesser extent, solvent acidity:
G = 102.96(±9.02)SPP + 78.20(±27.78)SA - 0.35(±6.28) [10.3.20]
with r = 0.965 and sd = 7.6.
It should be noted that the independent term for the gas phase is very small and highly
imprecise in both the G scale and the Φ scale.