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π Molecular Orbitals of 1,3-Butadiene

butadiene MOs generated from ethene MOs
The diagram to the right shows the relative energies of the π molecular orbitals of 1,3-butadiene (derived from ethene) and the electron configuration. 

The horizontal center line denotes the energy of a C atomic p orbital. Orbitals below that line are bonding those above are anti-bonding.

We now have 4 electrons to arrange, 1 from each of the original atomic p orbitals. These are all paired in the two stabilised pi bonding orbitals, π1 and π2.  The highest occupied molecular orbital or HOMO is π2 in 1,3-butadiene (or any simple conjugated diene).

In contrast, the anti-bonding π* orbitals contain no electrons. The lowest unoccupied orbital or LUMO  is π3 in 1,3-butadiene (or any simple conjugated diene).


Pi molecular orbital diagram for 1,3-butadiene

The relative energies of these orbitals can be accounted for by counting the number of bonding and anti-bonding interactions in each:

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary