Chemistry Reference
In-Depth Information
D
E
exch-ov
exchange-overlap component (10
3
E
h
) of the bond energies for
the ground state of simple homonuclear diatomics
Table 9.3
2
g
1
g
2
u
R
/
a
0
H
2
He
2
ð
Þ
H
2
ð
Þ
ð
Þ
S
S
S
1
58.091 3
104.432 1
529.288 2
84.918 6
119.027 4
1.2
229.410 1
98.085 9
119.626 4
1.4
61.884 6
102.805 7
112.541 5
26.328 8
1.6
1.8
102.349 4
101.597 1
68.078 4
2
98.766 2
89 016 2
83.282 4
3
66.453 9
34.683 1
46.417 8
4
37.676 4
9.680 6
13.793 0
5
19.258 7
2.205 8
3.420 7
6
9.087 9
0.449 83
0.783 32
with the internuclear distance R as exp(
c
0
R) and being always repulsive
for H
2
. This means that, classically, no bond can be formed between
a ground-state H atom and a protonH
þ
. For the remaining systems,
D
E
cb
is repulsive at small values and attractive at large values of R, but always
insufficient to originate a sufficiently stable chemical bond. For all
systems, the greater part of the bond energy comes from the exchange-
overlap contribution between spherically polarized charge distributions
(Tables 9.3 and 9.4), which, in the bond region, is attractive for H
2
,H
2
and He
2
and repulsive for the He-He interaction.
Hence, we conclude that, for all chemically bonded molecules, the
dominant component of the interaction in the bond region is the
exchange-overlap component, which depends on the spin coupling of
the interacting atoms; this is always attractive when forming the chemical
bond (Table 9.3) and is repulsive when bonding is forbidden (Table 9.4).
So, the exchange-overlap component is at the origin of both the chemical
bond and Pauli repulsion. In an appendix given as an electronic addition
Table 9.4
D
E Pauli repulsion and its Coulombic and exchange-overlap components
(10
3
E
h
) for the He(1s)-He(1s) interaction in the medium range of interatomic
separations
cb
exch-ov
R
/
a
0
DE
DE
DE
2
136.616
27.282 1
163.898
2.5
42.669 3
7.549 17
50.218 5
3
12.964 0
1.930 70
14.894 7
3.5
3.799 79
0.468 75
4.268 54
4
1.073 17
0.109 52
1.182 69
4.5
0.292 81
0 024 864
0.317 67
5
0.077 48
0.005 522
0.083 00
