Chemistry Reference
In-Depth Information
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Origin of the VdW bond in He
2
(
1
)
Figure 4.3
the quadrupole-quadrupole interactions (centrosymmetric molecules).
The latter are responsible for the typical T-structure observed for the
dimers of H
2
, N
2
and F
2
, as will be seen in Section 5.3 of the next
chapter.
An approximate non-expanded potential energy curve of the (H
2
O)
2
dimer evaluated by Magnasco et al. (1985) is schematically shown in
Figure 4.4. We see that the first-order interaction E
1
already shows, in this
range of internuclear distances, a minimum, chiefly due to the first-order
electrostatic component, while second-order interactions (induction
plus dispersion) simply deepen such a minimum, strengthening the bond.
It is appropriate in this case to speak of formation of a hydrogen bond,
essentially electrostatic in origin. It is of interest to notice the change 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
Origin of the hydrogen bond in (H
2
O)
2
(
1
A
1
)
Figure 4.4
