Dealing with the Solvent - Molecular Modelling - page 368

M
Figure 23.3 The Onsager model
In any continuum model, M resides in a cavity carved out of an infinite slab of continu-
ous dielectric (the solvent), as shown in Figure 23.3. The cavity is a basic concept in all
continuum models; the size and shape of the cavity are differently defined in the various
versions of continuum models but, as a general rule, it should exclude the solvent and con-
tain within its boundaries the largest possible part of the solute charge distribution. Much
attention has been paid to the portion of the solute electronic charge outside the boundary
of the cavity and the term escaped charge is often used in this context.
It is universally accepted that the cavity shape should ideally reproduce the molecular
shape but as usual in modelling studies there is a trade-off between computer resource
and the desire for better accuracy. Calculations are usually easiest when the cavity has a
simple shape.
23.3.1 The Onsager Model
In the simplest such model, named after Onsager, the cavity is a sphere of fixed radius; the
relative permittivity is 1 inside the cavity and equal to that of the solvent outside. M must
have a nonzero dipole moment, which induces a polarization P in the dielectric. P then
reacts with the solute dipole, leading to stabilization. Workers in the field speak about the
self-consistent reaction field (SCRF) .
A suitable Onsager sphere radius for (for example) phenylanine can be found from a
Gaussian 03 run such as:
%chk=d:\phenopt.chk
# B3LYP/6-311G ∗∗ Opt geom=check volume
L-phenylanine geometry optimization then volume
01
which produced the following MC calculation of the molecular volume, and estimate of
the cavity sphere radius.
Using the total density.
Monte-Carlo method of calculating molar volume:
based on 0.001 e/bohr ∗∗ 3 density envelope.
Number of points per bohr ∗∗ 3 = 20 CutOff= 1.00D-04
Using the SCF density.
There are 379 points. Will hold 379 in memory.