Chemistry Reference
In-Depth Information
Geometry
8
of the
ð
H
2
O
Þ
2
linear water dimer
Figure 5.6
5.3 THE ELECTROSTATIC MODEL OF THE
HYDROGEN BOND
Buckingham and Fowler (1983) asked whether electrostatic interactions
could predict structures of van der Waals molecules. The answer is 'yes
they can', not only for hydrogen-bonded systems but generally for VdW
complexes. All considerations we made before, in fact, led us to propose
a simple electrostatic model for the elementary prediction of the angular
geometries of VdWdimers based on the characterization of a molecule by
just its first two observable electric moments (Magnasco et al., 1990a).
The relative stability of different angular geometries of 35 VdW dimers
was correctly predicted just from the pictorial analysis of the electrostatic
interactions between these moments. We enter in some detail in the
following.
For small molecules (such as HF, H
2
O, NH
3
, CO, H
2
and N
2
) the
electrostatic model is based on the following rules. It is assumed that
molecules in the dimer be kept a distance R apart not less than the sum of
their Van der Waals radii.
1. Each molecule is characterized by its first two observable electric
moments (quadrupole alone for centrosymmetric molecules, terms
higher than R
5
not being considered in the formulation of the
model).
2. Qualitative evaluation (attractive or repulsive) is made of the
dipole-dipole
R
3
ð
Þ
, dipole-quadrupole plus quadrupole-dipole
ð
R
4
Þ
ð
R
5
Þ
electrostatic interactions
for the different angular geometries of the dimer.
, and quadrupole-quadrupole
8
Angle
a
in this figure is the supplement of angle
u
B
of Table 5.3d.
