Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 609
They found the frequency of this vibration in DMSO to be shifted by more than 150 cm -1
and to be correlated to the Gutmann acceptor number (AN) for the solvents. Based on our
analysis, the frequency shift reflects the acidity and, to a lesser extent, the polarity of the solvent,
according to the following equation:
v S=O = 38.90(±7.50)SPP - 89.67(±4.60)SA + 1099.1 [10.3.22]
with n = 21, r = 0.979 and sd = 6.8 cm -1 .
This fit is very good, taking into account that it encompasses highly polar solvents
such as DMSO itself, highly acidic solvents such as trifluoroacetic acid (SPP=1.016,
SA=1.307), and highly non-polar and non-acidic solvents such as the gas phase.
Giam and Lyle 84 determined the solvent sensitivity of the 19 F NMR shifts on
4-fluoropyridine relative to benzene as an internal reference in 31 of the solvents listed in
Table 10.3.1. If DMF is excluded on the grounds of its odd value, the chemical shifts for the
remaining 30 solvents are accurately described by the following function of solvent acidity
and polarity:
Δ 4F = -1.66(±0.57)SPP - 6.84(±0.36)SA - 6.50(±0.45) [10.3.23]
with r = 0.971 and sd = 0.34 ppm.
Clearly, the acidity of the medium is the dominant factor in the chemical shift measured
for this compound, which reveals the central role played by the lone electron pair in
pyridine.
Both absorption and emission electronic transitions are acceptably described by our
scales, as shown by a recent study 85 on the solvation of a series of probes containing an
intramolecular hydrogen bond, so no further comment is made here other than the following:
even if one is only interested in evaluating the change in dipole moment upon electronic
excitation via solvatochromic analysis, a multi-parameter analysis must be conducted in order
to isolate the shifts corresponding to the pure dipolar effect of the solvent. 85
Lagalante et al. 86 proposed the use of 4-nitropyridine N-oxide as a suitable
solvatochromic indicator of solvent acidity. The hypsochromic shifts determined by these
authors for 43 of the solvents in Table 10.3.1 are due largely to the acidity of solvent and, to
a lesser extent, also to its basicity; based on the following equation, however, solvent polarity
induces a bathochromic shift in the band:
v max = -0.92(±0.44)SPP + 0.68(±0.17)SB + 3.63(±0.23)SA +29.10(±0.31) [10.3.24]
with r = 0.943 and sd = 0.31 kK.
As can be seen from Figure 10.3.7, this probe classifies solvents in groups encompassing
non-acidic solvents (below 29 kK), moderately acidic solvents (at about 30 kK) and
highly acidic solvents (between 31 and 32 kK). Because the solvatochromism does not
change gradually with increase in solvent acidity, the probe appears to be unsuitable for
quantifying this effect.
Davis 87 determined the solvatochromism of a charge-transfer complex formed by
tetra-n-hexylammonium iodide-nitrobenzene in 23 different solvents and, using the Z and
E T(30) scales, observed a bilinear behavior in scarcely polar and highly polar solvents. A