Chemistry Reference
In-Depth Information
Table 2.6
Comparison between vertical UV ionization potentials and theoretical
MO-SCF results (eV) for the A
g
ground states of diborane and ethylene
MOs
B
2
H
6
MOs
C
2
H
4
Experimental
a
Theoretical
b
Experimental
c
Theoretical
b
1b
2g
11.89
13.25
1b
2
u
10.51
10.09
3a
g
13.30
14.56
1b
2g
12.46
13.77
1b
3u
13.91
15.24
3a
g
14.46
15.28
1b
2
u
14.75
15.68
1b
3u
15.78
17.52
2a
u
16.11
17.86
2a
u
18.87
21.29
a
Lloyd and Lynaugh 1970.
b
Palke and Lipscomb 1966.
c
Branton et al. 1970.
by Palke and Lipscomb (1966) assuming the validity of Koopmans'
theorem:
I
i
«
i
ð
2
:
56
Þ
Even if the quantitative agreement between the two sets of data is rather
unsatisfactory,
10
nonetheless both experiment and calculation show that
the
p
bonding level in both molecules is:
(
«ð
1b
2u
Þ¼«
p
14
:
5 eV for diborane
ð
2
:
57
Þ
«ð
Þ¼«
p
:
1b
2u
11
0 eV for ethylene
so that we can say that the two protons entering the three-centre double
bonds in diborane stabilize the double bond, the
energy level in diborane
now appearing as the fourth ionization potential instead of the first,
as observed in ethylene.
p
2.5 THE HETEROPOLAR BOND
In the following, we extend our method to consideration of the hetero-
polar chemical bond. New aspects are now that: (i) both atomic energy
difference
a
2
a
1
and bond integral
b
do contribute to the bond energy,
often being of the same order of magnitude; and (ii) the molecular charge
distribution is asymmetric so that it generates an electric dipole moment.
Assuming for simplicity orthogonal AOs (remember Footnote 3), the
10
Not accounting for any correlation energy, theoretical MO calculations heavily overestimate
ionization potentials.
