Chemistry 401

Molecular Orbital Theory

Experimentally, O₂ is found to have two unpaired spins – VBT can not explain a simple molecule like O₂ !

 

Linear Combination of Atomic Orbitals

Assumptions:

1) Molecular Orbitals look like atomic orbitals, especially near the nucleus

2) Molecular Orbitals are constructed by interference patterns of atomic orbitals

3) The total number of molecular orbitals = the total number of atomic orbitals

4) Molecular wavefunctions are products of the molecular orbital functions

 

Overlap of atomic orbitals

constructive interference gives bonding orbital, no nodes

destructive interference gives antibonding orbital one node, perpendicular to the internuclear axis

 

 

 

 

 

 

molecular orbitals are labeled by nodal properties:

0 nodes parallel to the internuclear axis: σ

1 node parallel to the internuclear axis: π

2 nodes parallel to the internuclear axis: δ

0 nodes perpendicular to the internuclear axis: bonding

1 node perpendicular to the internuclear axis: antibonding

 

Construction of Energy Level diagrams

1) Define the basis set (i.e., the set of atomic orbitals to be used, generally the valence orbitals)

2) # of MOs = # of AOs

3) The change in energy is proportional to the degree of overlap

4) Better overlap occurs between AOs of about the same energy

5) Bonding orbitals lower energy, antibonding orbitals raise energy relative to the AOs

 

Bond Order = BO = ½(# electrons in bonding orbitals - # electrons in antibonding orbitals)

 

H₂

BO = 1, same as VBT

 

First row diatomics basis set : 2s and 2p (1s is a core orbital, so will not be affected much by bonding)

A₂configurationB.O. predicted spinsobserved spins

 

 

Li₂1σ²1 00

 

Be₂1σ²1σ*² 000

 

B₂1σ²1σ*²1π² 122

 

C₂1σ²1σ*²1π⁴ 200

 

N₂1σ²1σ*²2σ²1π⁴ 300

 

O₂1σ²1σ*²2σ²1π⁴1π*² 222

 

F₂1σ²1σ*²2σ²1π⁴1π*⁴ 100

 

Ne₂ 1σ²1σ*²2σ²1π⁴1π*⁴2σ*² 000