- Why is parameterizing a continuum solvent model with the conventional dielec-
tric constant possibly physically unrealistic?
The conventional dielectric constant is an experimental quantify that refers to
the solvent as a continuous insulating medium. On the molecular scale solute
and solvent are not separated by a smooth medium, but rather by discrete
particles (molecules) with empty interstices. - Consider the possibility of parameterizing a continuum solvent model with
dipole moment.
Continuum solvent models are normally parameterized with the solvent dielectric
constant (but see the COSMO models, Chapter 8). First we note that dielectric
constant and dipole moment are not in general well correlated; from Chapter 8:
For 24 solvents encompassing nonpolar (e.g. pentane,m0.00,e1.8), polar aprotic (e.g.
dimethyl sulfoxide,m3.96,e46.7), and polar protic (e.g. water,m1.85e80) dispositions,
the correlation coefficientr^2 ofewithmwas only 0.36 (removing formic acid and water
raised it to 0.75). For nine nonpolar, seven polar aprotic, and 8 polar protic solvents,
considered as separate classes,r^2 was 0.90, 0.87, and 0.0009 (sic), respectively .....
If we consider just essentially using dipole moment as a surrogate for dielectric
constant, with minor conceptual adjustments like some changes in the parameteri-
zation constants, then from the above, for nonpolar and polar aprotic solvents the
correlation is good enough that it may be possible to parameterize with dipole
moment, but there is no clear indication that this would have any advantage.
Furthermore, water, the most important solvent, belongs to the polar protic class,
for which there is no correlation.
Less clear is whether a different approach than that used with dielectric constant
might be fruitful with dipole moment. A useful solvation algorithm does not seem to
have emerged from studies of the effect of dipole moment on solvation energies, e.g.
(a) Effect of bond and group dipole moments on the enthalpy of solvation of
organic nonelectrolytes: Antipin IS, Karimova LKh, Konovalov AI (1990) Z
Obshch Khim 60:2437–2440. (In Russian)
(b) Free energy of solvation of aromatic compounds and their polarizability:
Gorbachuk VV, Smirnov SA, Solomonov BN, Konovalov AI (1988) Dokl
Akad Nauk SSSR 300:1167. This paper studied dipole moment as well as
polarizability. (In Russian)
Chapter 8, Harder Questions, Answers
Singlet Diradicals
- Is CASSCF size-consistent?
We saw thatfull CIis size-consistent (Section 5.4.3). Now, CASSCF iscomplete
CI,within a specified set ofmolecular orbitals. If done right it is size-consistent.
648 Answers