Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 21
For the calculation of the electrostatic term the system under study is characterized by
two dielectric constants, in the interior of the cavity the constant will have a value of unity,
and in the exterior the value of the dielectric constant of the solvent. From this point the total
electrostatic potential is evaluated. Beyond the mere classical outlining of the problem,
Quantum Mechanics allows us to examine more deeply the analysis of the solute inserted in
the field of reaction of the solvent, making the relevant modifications in the quantum mechanical
equations of the system under study with a view to introducing a term due to the
solvent reaction field. 38 This permits a widening of the benefits which the use of the continuum
methods grant to other facilities provided for a quantical treatment of the system, such
as the optimization of the solute geometry, 39 the analysis of its wave function, 40 the obtaining
of its harmonic frequencies, 41 etc.; all of which in the presence of the solvent. In this
way a full analysis of the interaction solute-solvent can be reached at low computational
cost.
Continuum models essentially differ in:
• How the solute is described, either classically
or quantally
• How the solute charge distribution and its
interaction with the dielectric are obtained
• How the solute cavity and its surface are
described.
The first two topics are thoroughly considered
in Chapter 8 by Prof. Tomasi, here we will
consider in detail the last topic.
2.1.3.3. Cavity surfaces
Figure 2.1.5. Molecular Surface models. (a) van
der Waals Surface; (b) Accessible Surface and
(c) Solvent Excluding Surface.
Earliest continuum models made use of oversimplified
cavities for the insertion of the solute in the
dielectric medium such as spheres or ellipsoids. In
the last decades, the concept of molecular surface
as become more common. Thus, the surface has
been used in microscopic models of solution. 38,42
Linear relationships were also found between molecular
surfaces and solvation free energies. 43
Moreover, given that molecular surfaces can help
us in the calculation of the interaction of a solute
molecule with surroundings of solvent molecules,
they are one of the main tools in understanding the
solution processes and solvent effects on chemical
systems. Another popular application is the generation
of graphic displays. 44
One may use different types of molecular
cavities and surfaces definitions (e.g.
equipotential surfaces, equidensity surfaces, van
der Waals surfaces). Among them there is a subset
that shares a common trait: they consider that a
molecule may be represented as a set of rigid interlocking
spheres. There are three such surfaces: a)