Molecular Orbitals: Understanding & Rules

[nigelscott]

My general understanding of Molecular Orbitals is as follows:

N atoms come together to form N molecular orbitals.

N/2 of the orbitals will be spatially asymmetric with symmetric spins.This corresponds to the antibonding orbital.

N/2 will be spatially symmetric with antisymmetric spins. This corresponds to the bonding orbital..

Each bonding orbital can contain a maximum of 2 electron with opposite spins.

Each antibonding orbital can either be empty or contain 1 electron since parallel spins in the same state are not allowed and with opposite spins the overall orbital would be symmetric which is not allowed for a fermion.

Excited electron can move to both higher energy bonding and antibonding orbitals as long as the Pauli Exclusion principle is not violated.

Is this the correct interpretation (particularly the last paragraph)?

 

[DrDu]

Well, also anti-bonding orbitals can be occupied with 2 electrons whose spin has to be antiparallel then. On the other hand, bonding orbitals can also be empty.

 

[nigelscott]

But if the spins are opposite in the anti-bonding case doesn't the overall wave function become symmetric which is not allowed for Fermions.

 

[DrDu]

If you have a product of two identical orbitals, this is always symmetric and you have to combine it with an antisymmetrric spin function. Could it be that you mixed this up with valence bond theory?

 

[nigelscott]

Yes, I think you are right. I think the thing that is throwing me is the usage of symmetric and antisymmetric. I understand this for AOs but maybe a better way to look at this for MOs is to replace these terms with constructive and destructive interference. Now what you are saying makes perfect sense. The bonding orbitals get filled first followed by the anti-bonding orbitals with the molecular stability being determined by the Bond Order.

I am not a chemist but a retired EE who is re-learning semiconductor band theory for fun!