[Kalibasa]

I'm looking at an energy diagram of CO, and I understand the important point (how the contributions of the two atoms aren't equally distributed- the higher-energy carbon contributes more to the higher-energy, antibonding MO's).  I'm confused, though, about the numbers used before the pi's and sigma's. Here is the diagram:

4σ*
2π*
3σ
1π
2σ*
1σ

I'm guessing the numbers beforehand correspond exactly to the basic energy levels we learned in intro chem (ex. n=1, n=2), and that here the energy levels are "out of order" (ex. 3 before 2) because the antibonding orbitals are that much higher in energy than bonding ones. But I don't understand why 1π is higher than 2σ*; if they both have one node, shouldn't the n=2 orbital be higher?

I also don't understand why these numbers are used here but weren't used before, for the homonuclear diatomic molecules. N₂, for instance, has the exact same orbital progression as CO, but there we didn't list energy levels (it was just σ, σ*, etc). Why do we list them here?

Thanks!

[KritikalMass]

Hmm, this was a difficult question! I looked all over the net and in my old textbooks for about 2 hours and couldn't find a good answer. I am unable to help you out, but will post a couple of pics so that maybe someone can have a better idea of what you are referring to.

[jj74]

I'm not sure, but maybe it's a simple progressive number:
lower Sigma:1, the first sigma higher 2 and so on..
the same for the Pi orbitals; you could try asking in the chemical physics forum

[cth]

You can have a look at this webpage http://courses.chem.psu.edu/chem210/mol-gallery/co/co.html for a nice explanation

4σ*
2π*
3σ
1π
2σ*
1σ

To put it simply:
The 1π and 2π* orbitals are located more onto the oxygen atom, which is more electronegative than the carbon. These orbitals are lowered in energy.
The 3σ and 4σ* orbitals are more onto the carbon atom. So, it is a bit higher in energy.
Overall, the σ orbitals become higher in energy than the π.

[KritikalMass]

Hey thanks cth! I'm not the OP but glad someone came along to give an answer- that question was buggin' me like crazy!

[Kalibasa]

Thank you all

[soot]

They are merely markers to separate the sigma from pi.

ie if you are using symmetry to form MOs)

1a1 (sigma)
2a1 (sigma star)

They are sequential and not necessarily a reflection of the principal quantum number.