MO Diagrams for
Diatomic Molecules
Chapter 5
Friday, October 9, 2015
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MO Diagrams for Diatomic Molecules: Understanding Bonding and Orbital Interactions, Study notes of Chemistry
The use of MO (Molecular Orbital) diagrams to understand the electronic structures and bonding in diatomic molecules. Topics include homonuclear and heteronuclear diatomic molecules, orbital interactions, bond orders, and MO energies. The document also discusses the importance of symmetry in treating larger molecules within the LCAO-MO (Linear Combination of Atomic Orbitals - Molecular Orbitals) theory framework.
What you will learn
- How do homonuclear and heteronuclear diatomic molecules differ in their MO diagrams?
- What factors control the internuclear distance in diatomic molecules?
- What role does symmetry play in treating larger molecules within the LCAO-MO theory framework?
- How do orbital interactions and mixing affect the energies and bonding in diatomic molecules?
- What is LCAO-MO theory and how is it used to predict the electronic structure of molecules?
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MO Diagrams for
Diatomic Molecules
Chapter 5
Friday, October 9, 2015
Homonuclear Diatomic Molecules
What happens when we move to more complicated systems?Consider O
.
-^
The Lewis dot structure famously predicts the wrong electronicstructure for O
-^
We can use LCAO-MO theory to get a better picture:
s^ a
p
a
s^ b
p
b
Notice that
E
E
,
because theσ bonds havemore overlapthan π bonds
E
σ^
E
π
Orbital Mixing
Orbitals of similar but unequal energies can interact if they have thesame symmetryThe 2
s
and 2
p
z
orbitals form MOs with the same symmetry (
σ
g
and
σ
u
).
sp mixing
causes the
σ
g^
and
σ
u
MOs to be pushed apart in energy:
The σ andπ orbitals change order!
Orbital Mixing
The size ofthe effectdepends onthe 2s-2penergydifference.
small
Z
eff
=
small energydifference = large
sp
mixing
large
Z
eff
=
large energydifference = small
sp
mixing
order changes
Bond Order and Bond Distance
The MO bond order is the main factor controlling the internucelardistance.
B
2
σ
2 σ*
2 π
2
BO = 1
C
2
σ
2 σ*
2 π
4
BO = 2
N
2
σ
2 σ*
2 π
4 σ
2
BO = 3
O
2
σ
2 σ*
2 σ
2 π
4 π*
2
BO = 2
F
2
σ
2 σ*
2 σ
2 π
4 π*
4
BO = 1
O
- 2
σ
2 σ*
2 σ
2 π
4 π*
3
BO = 1.
O
+ 2
σ
2 σ*
2 σ
2 π
4 π*
1
BO = 2.
N
+ 2
σ
2 σ*
2 π
4 σ
1
BO = 2.
C
2– 2
σ
2 σ*
2 π
4 σ
2
BO = 3
Relative AO Energies for MO Diagrams
Photoelectron spectroscopy
gives us a pretty good idea of the relative
energies for AOs.
H
He
Li
Be
B
C
N
O
F
Ne
B
C
N
O
F
Na Ne
Mg
Al
Si
P
S
Cl
Ar
Al
Si
P
S
Cl
Ar
s
2 s
p
s
p
–13.6 eV
–18.6 eV –40.2 eV
Relative AO Energies for MO Diagrams^ H
He
Li
Be
B
C
N
O
F
Ne
B
C
N
O
F
Na Ne
Mg
Al
Si
P
S
Cl
Ar
Al
Si
P
S
Cl
Ar
s
2 s
p
s
p
–19.4 eV
–15.8 eV
–32.4 eV
–10.7 eV
Photoelectron spectroscopy
gives us a pretty good idea of the relative
energies for AOs.
Energy
Heteronuclear Diatomic Molecules: CO
In molecules with more than one type of atom, MOs are formed fromAOs that have different energies. Consider CO:
2 s
a
p
a
C
s b
2 p
b
O
C≡O
σ
σ*
π
π* σ
σ*
Bonding orbitals
get polarized
towards oxygen
Anti-bonding orbitalsget polarized towards
carbon
HOMO is on carbon
LUMO is on carbon too!