Hello all,

I am hoping that someone on this forum might have a bit of chemistry under their belt to lend a hand.

I am having trouble with the concept of molecular orbitals. I understand that atomic orbitals form them and there is a conservation of orbitals but I don't understand the physical nature of anti-bonding molecular orbitals. My prof is telling me to think about wave interference, but it doesn't really put a nice 3-D orbital picture in my head. I know how waves function, but I don't see the relation to orbitals.

Another thing that is confusing me, is why in the molecular bonding diagram for O2 the 2pi and 2sigma differ in relative energy levels with orientation of the corresponding antibonding 2pi* and 2sigma*.

Thanks a bunch!

 

Visualization of MOs is restricted to a few simple cases of homo-nuclear diatomic molecules. So try not to get too caught up in that. Only the simplest cases can be calculated or visualized at all.

Also, bonding theory is just a model. Some of the nomenclature used is not entirely accurate, but just helps hang a label on an observed phenomena. To say that anti-bonding orbitals are caused when the electrons in the atomic orbitals are out-of phase is kind of like saying that two electrons can co-exist in the same orbital as long as their spin is different. Spin is just an name given to a particular state property, as is phase.

When two atoms get close enough that their electron orbitals interact, the orbitals are said to split into two MOs. They call these bonding and anti-bonding orbitals. The lower energy MO increases the attraction between the nuclei and the higher energy MO decreases it, hence the names.

The whole phase relationship is confusing but, like spin, gives us a away to account for observation. For instance, when two hydrogen atoms get close enough to bond, the 2 electrons in the system fill the bonding MO. However, when two helium atoms get close enough, 2 of the 4 electrons will fill the bonding MO, while the other two must fill the anti-bonding MO. The attraction from the bonding MO is cancelled out by the anti-bonding MO so that helium does not form homo-nuclear diatomic molecules. Its not that some have phase issues and some do not such that some bond and others do not.

On your O2 question. I am not sure I fully understand what you are asking, but ... first, the corresponding anti-bonding orbital is always a higher energy orbital than the bonding orbital. So this is the case for all MOs, everywhere in the molecule. Also, are you referring to the MOs that arise from the 2p atomic orbitals? 2p orbitals have two lobes, so they can align linearly or in parallel. The linear alignment results in the (2) 2p-sigma and 2p-sigma* MOs and the parallel alignment results in the (4) 2p-pi and 2p-pi* MOs.

If you think diatomic oxygen is complicated, just try to wrap your head around something seemingly simple like ferric oxide, then try hemoglobin.