CHEM 2060 Lecture 27; MO N2 L27-1
MOs of Homonuclear Diatomics Li2 through N2 are not the same as O2 & F2
Li2 → N2
O 2, F 2
CHEM 2060 Lecture 27; MO N2 L27-2
How do we know there is a difference?
B2 is paramagnetic (10 electrons in total).
Only explicable using this MO scheme
Photoelectron spectroscopy (PES) supports the
MO schemes of N2 and O2, which are different
from one another.
WHY is N2 different from O2 ?
Both s and pz AOs mix to give σ MOs.
Whether or not AOs mix to give MOs depends
on:
1. symmetry (e.g., σ or π type interactions)
2. how close s and p orbitals are in energy!!
3. amount of overlap (proximity)
ΔE2s-2p increases across the period!!
CHEM 2060 Lecture 27; MO N2 L27-3
When do atomic orbitals mix to give molecular orbitals?
The key here is that the 2s – 2p energy gap (ΔE2s-2p) dictates how strongly the
orbitals interact.
Orbitals interact only when they are
(a) of the same symmetry
(b) are close in energy
(c) are spatially close to one another (overlap is not negligible)
As we go across a period (left to right), the 2s orbitals feel the additional nuclear
charge (increasing Z* across a period) to a greater extent than the 2p.
(Why? Remember SHIELDING AND PENETRATION! Go back and look at
the radial wavefunctions).
It is a matter of degree really. There IS a contribution from both the 2s and 2p
for all cases – it’s just too small for O2 and F2 to have an effect on the energy
ordering of the MOs. – as a result, we can treat the 2s and 2p separately for O2
and F2.
CHEM 2060 Lecture 27; MO N2 L27-4
How do we treat Li2 through N2? … how do we mix both s and p orbitals?
For the binuclear species Li2, Be2, B2, C2 and N2,
the 2s and 2pz orbitals…
…are of the correct symmetry to mix (all σ)
…are close enough in energy to mix
MIX 4 atomic orbitals (2 x 2s and 2 x 2pz) ⇒ GET 4 molecular orbitals
- less bonding
- raised energy
- less e- density between nuclei
- less antibonding
- lowered energy
- less e- density between nuclei
No mixing of s and p
Mixing of s and p
CHEM 2060 Lecture 27; MO N2 L27-5
O 2, F 2
Li2, Be2, B2, C2, N2
CHEM 2060 Lecture 27; MO N2 L27-6
This modifies our ideas about bonding and antibonding orbitals.
CHEM 2060 Lecture 27; MO N2 L27-7
MOs for homonuclear diatomics: electron filling
CHEM 2060 Lecture 27; MO N2 L27-8
MOs explain well the following trends: