The solvatochromism of
several polar solutes, including some that contain both hydrogen bond-donating
and
-accepting properties (coumarins 1, 102, 120, 151, 152, and 153; nile
red; and 4-aminofluorenone), is analyzed
in terms of three models: the Reichardt single parameter E
T
N polarity scale, the multiparameter Kamlet−Taft
equation, and the reaction field model. We use a “step-forward”
procedure to determine which terms of the
Kamlet−Taft equation are statistically significant in fitting
the data. These equations provide the best fits to
the data in almost all cases. We also find a correlation between the
parameters s and a, which quantify
the
effects on the transition energy related to the solvatochromic parameters
π* and α, respectively. This relationship
suggests that the magnitude of a is not indicative
of the strength of the hydrogen-bonding interaction, but
rather reflects the additional field produced from the dipole moment
of a hydrogen bond-donating molecule
that is held in an orientation that roughly parallels the solute dipole.
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