Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 751
empirical parameters are highly attractive for theoreticians
for testing their computational models and
improving their predictive power. At present, the
solvatochromic scales are under considerable scrutiny.
Thus, in a recent thermodynamic analysis,
Matyushov et al. 55 analyzed the two very popular polarity
scales, E T(30) and π*, based on the solvent-induced
shift of electronic absorption transitions
(Scheme 13.1.2)
Solvatochromism has its origin in changes in both dipole moment and polarizability of
the dye upon electronic excitation provoking differential solvation of the ground and excited
states. The dipole moment, μ e, of the excited state can be either smaller or larger than
the ground state value μ g. In the former case one speaks about a negatively solvatochromic
dye such as betaine-30, whereas
Table 13.1.4. Dye properties used in the
calculations. Data are from reference 55
Molecular parameter Betaine-30 4-Nitroanisole
Vacuum energy gap (eV) 1.62 4.49
R0 (Å) 6.4 4.5
αg (Å) 68 15
Δα (Å) 61 6
mg (D) 14.8 4.7
me (D) 6.2 12.9
Δμ (D) -8.6 +8.2
the π* dye 4-nitroanisole is positively
solvatochromic. Thus, polar
solvent molecules produce a red
shift (lower energy) in the former
and a blue shift (higher energy) in
the latter. On the other hand,
polarizability arguably always increases
upon excitation. Dispersion
interactions, therefore, would
produce a red shift proportional to
Δα = α e - α g of the dye. In other
words, the excited state is stabilized
through strengthening of
dispersive coupling. Finally, the
relative contributions of dispersion
and dipolar interactions will
depend on the size of the dye molecules with dispersive forces becoming increasingly important
the larger the solute. Along these lines, the dye properties entering the calculations
are given in Table 13.1.4. The purpose of the analysis was to determine how well the description
in terms of “trivial” dipolar and dispersion forces can reproduce the solvent dependence
of the absorption energies (and thereby, by difference to experiment, expose the
magnitude of specific forces),
�ωabs where
= Δ+ ΔErep + ΔEdisp + ΔEdipolar + Δ Ess
[13.1.12]
�ωabs absorption energy
Δ vacuum energy gap
ΔErep shift due to repulsion solute-solvent interactions (taken to be zero)
ΔEdisp shift due to dispersion interactions
ΔEdipolar shift due to dipolar forces of permanent and induced dipoles
ΔEss solvent reorganization energy
For the detailed and arduous calculation procedure, the reader may consult the paper
cited. Here, let us just make a few general comments. The solvent influence on
intramolecular optical excitation is treated by implementing the perturbation expansion