Handbook of Solvents - George Wypych - ChemTech - Ventech!

11.2 Dielectric solvent effects 681
a thin slab cavity,s=1inthenormal direction
ands=0inplane; and for a long cylindrical
cavity, s=0inthelongitudinal axis
and s = 1/2 in the transverse direction. The
shape of the ellipsoidal cavity is supposed
to be primarily determined by the shape of
the solute molecule. Typical cases are long
polyenes and large planar aromatic hydrocarbons.
One can assume s = 0 for the
strong π→ π*
absorption bands of these
molecules. For smaller molecules, however,
one should assume s ≈ 1/3 regardless
of the shape of the solute molecule, as the
cavity shape then may be primarily determined
by the solvent molecules rather than
the solute molecule. Note that Eq. [11.2.2]
gives the Chako formula for s = 1/3, i.e., for
the spherical cavity.
For transitions whose moments are in the longitudinal axis of a long cylindrical cavity
or in the plain of a thin slab cavity, Eq. [11.2.2] withs=0leads to f ′′ /f = 1/n, so that the absorption
intensity always decreases with the refractive index. If the transition moment is
normal to a thin slab cavity, Eq. [11.2.2] withs=1leads to f ′′ /f = n 3 Figure 11.2.1. Dependence of f ′ / f on the refractive index
n for different values of s. [After reference 6]
. The dependence of
the ratio f ′′ /f on the refractive index n according to Eq. [11.2.2] is illustrated for different
values of s in Figure 11.2.1. The slope of the ratio is always positive for s > 1/4. For0