Chemistry Reference
In-Depth Information
Δ
E
/10
-3
E
h
E
1
(
H
2
O
)
2
1
A
1
5.4
0
R
/
a
0
~
E
1
+
E
2
Figure 11.9 Origin of the hydrogen bond in (H
2
O)
2
(
1
A
1
)
in the scale factor for energy, from 10
6
E
h
for He
2
(VdW bond) to
10
3
E
h
for (H
2
O)
2
(H-bond), even though roughly in the same region of
intermolecular distances (5.6a
0
for He
2
and 5.4a
0
for the H
2
O dimer).
On these grounds, some time ago, Magnasco et al. (1990a) derived a
simple electrostatic model for VdW complexes, where the angular geo-
metry of the dimers was predicted in terms of just the first two observable
electric moments of the monomers. The model allowed for the successful
prediction of the most stable angular shapes of 35 VdW dimers.
11.5 THE KEESOM INTERACTION
Keesom (1921) pointed out that if two dipolar molecules undergo thermal
motions, then they will on average assume orientations leading to attrac-
tion according to
C
6
ð
T
Þ
R
6
E
6
ð
Keesom
Þ¼
ð
11
:
61
Þ
C
6
ð
T
Þ
being the T-dependent coefficient:
2
A
B
2
m
m
C
6
ð
T
Þ¼
ð
11
:
62
Þ
3kT
