Chemistry Reference
In-Depth Information
-6
E
h
Δ
E
/10
E
1
Σ
+
1
He
2
5.6
0
R
/
a
0
~
disp
E
1
+
E
2
S
g
)
Figure 11.8 Origin of the VdW bond in He
2
(
1
should be remarked that, in this region of internuclear distances, both
electrostatic and induction energies are negligible because of their charge-
overlap nature. For simple 1s STOs on each He atom, with orbital
exponent c
0
, in fact (
r ¼
c
0
R)
¼
E
e
1
¼
4c
0
exp
ð
2
rÞ
5
8
r
3
4
r
1
6
r
D
E
cb
2
3
1
þ
ð
11
:
59
Þ
r
¼
2
X
i
2
jð
A
0
A
i
j
U
B
Þj
ind
2
ind
;
A
ind
;
B
E
¼
E
þ
E
ð
11
:
60
Þ
2
2
«
i
and, for the neutral atom, U
B
decreases exponentially far from the B
nucleus.
The situation is different for the long-range interaction of two ground-
state H
2
O molecules, whose potential energy curve is schematically
shown in Figure 11.9. In this case, the first-order interaction E
1
shows
in the medium range a minimum mostly arising from the dipole-dipole
interaction going as R
3
, and the second-order interaction simply dee-
pens this minimum, strengthening the bond. As already seen in the case of
(HF)
2
, it is appropriate in this case to speak of formation of a hydrogen
bond, essentially electrostatic in origin. It is of interest to note the change
