Why is the MO theory for O2 different from VB theory?

[richardlhp]

Hi,

just wondering about MO theory. Take O2 for example; O is sp2 hybridised according to VB theory. However, when drawing the MO diagram for O2, one considers the sigma overlap between the s orbital and p orbitals separately (according to the theory), rather than considering the overlap of sp2 orbitals and one pz pi overlap. Why is this so?

Thanks to anyone who can help!

Cheers!

 

[DrDu]

Who does consider O2 as sp2 hybridized in VB theory? That's nonsense.

 

[NEILS BOHR]

O is sp2 hybridised according to VB theory

who told you this??

 

[pseudophonist]

If you want to know what's really going on, you have to set up and solve the Schrödinger equation for the system. This is difficult and sometimes unsolvable, so we have some simpler models.

One is hybridization theory, which is the mixing of atomic orbitals to make hybrid orbitals for one atom. This is useful to explain the shapes and reactivity of some organic molecules amongst other things.

Another is Molecular Orbital theory which is the mixing of atomic orbitals of similar energy for two bonded atoms. This is useful to explain things like the paramagnetism of oxygen.

Don't mix the two unless you're using quantum mechanics to make a new theory.

Furthermore, the reason the other posters are saying that oxygen isn't sp2 hybridised is because MO theory is a better model for oxygen. One could argue that sp2 hybridised orbitals bear the closest resemblance to the MO theory model orbitals (particularly the antibonding orbitals), but this is pretty much beside the point.

 

[DrDu]

No, pseudophonist. It doesn't even make much sense in Valence Bond theory to assume Oxygen in O2 to be sp2 hybridized.


And, btw., you can also explain pretty well in valence bond theory why oxygen is paramagnetic as first shown already in 1937 by Wheland.

 

[pseudophonist]

True enough, I didn't know about Wheland. MO does make the prediction of oxygen's magnetic properties very easy to see though, while it was thought for a long time that VB theory couldn't predict that.

 

[DrDu]

Well, basically it is quite easy also in VB: Two 3-electron bonds are more stable than one 2-electron bond and one 4-electron bond. Then use Pauli principle.

 

[DrDu]

Btw., we know that the singulet state of the oxygen, which is prediced by a naive use of valence bond theory, corresponds to an excited state of the oxygen molecule. However, singulet oxygen is paramagnetic, too, due to the orbital momentum in the Delta state. The paramagnetism of Delta O2 is about 80 per cent of singulet oxygen. In fact, when you read early articles on MO-theory of oxygen, paramagnetism didn't come as a surprise.