Chemistry Reference
In-Depth Information
between the 1
σ
+
orbital shows more significant hybridization
on the C side of the bond than the O, since this MO is much closer in energy to the C(2s)
orbital than to the O(2s). The difference in electronegativities also means that the bonding
1
π
and 2
π
levels. In fact, the 5
π
orbitals have larger lobes on C than on O. The NO molecule has one additional electron to
CO and so this must occupy one of the degenerate 2
π
orbitals are polarized toward O. Correspondingly, the antibonding (and empty) 2
π
orbitals.
Problem 7.11:
The bond length of NO
+
(1.062 Å) is shorter than that of NO by 0.089
Å, whereas CO
+
has a bond length (1.115 Å) only
0.013 Å shorter than CO. Use the
MO diagram of Figure 7.32 to explain these observations.
−
7.6 More Complex Polyatomic Molecules
In Section 7.4 we considered the molecules formed by second-row elements with H. In
each case there was only one heavy atom situated at the junction of all the symmetry
elements, i.e. at the 'point' of the point group. This allows the symmetry labels for the
p-orbitals of this atom to be taken directly from the right-hand column of the standard
character table in Appendix 12. To form the MO diagram, we then considered the SALCs
for the H(1s) orbitals and matched symmetry labels to identify the MOs that will give
bonding-antibonding interactions.
In this section we introduce more complex molecules in which there are multiple heavy
atoms. We now need to identify SALCs for the sets of atoms that are related by symmetry
operations. The MO diagram is then constructed by matching irreducible representations
for the SALCs.
7.6.1 Ethene
As a first example we consider the
D
2h
molecule C
2
H
4
. The two C atoms form one set
of atoms and the four H atoms another. In the
D
2h
point group there are three mutually
perpendicular
C
2
axes, labelled
C
2
(
X
),
C
2
(
Y
) and
C
2
(
Z
). Figure 7.33 shows the axis system
used in this analysis, which places
X
along the C C bond and
Z
out of the molecular
plane. The figure also gives the basis of four H(1s) orbitals for the SALC construction for
this set of atoms.
C
2
(
Y
)
Y
s
1
s
2
X
C
2
(
X
)
s
4
s
3
C
2
(
Z
)
Z
Figure 7.33
The axis system and basis used to obtain SALCs for the H(1s) orbital set in the
D
2h
molecule ethene.
