Ch 10: Ethene MOs

Chapter 10: Conjugation in Alkadienes and Allylic Systems

π Molecular Orbitals of Ethene

In chapter 1 we saw that the molecular orbitals of H₂ are created by the combination of 1s orbitals.
The in-phase combination gave the bonding orbital.
The out-of-phase combination the anti-bonding orbital.
For ethene, the σ framework is created by the interaction of the sp² hybrid orbitals of the C atoms and H1s orbitals.
Each C has a p orbital unused by the hybrids and it is these on the adjacent C atoms that interact to form the C-C π bond.

The in-phase and out-of-phase combinations for the interaction of two p atomic orbitals in a π (i.e. side to side) fashion to give the two π molecular orbitals are shown below:

Here the different phases of the lobes in the π orbitals are represented by the two different colours.

Notice that neither of the molecular orbitals are symmetric with respect to the internuclear axis and that the out of phase combination has an extra vertical nodal plane between the nuclei.

p atomic orbital combinations to give the pi system molecular orbitals

QUESTION: What type of molecular orbitals are symmetric with respect to the internuclear axis ? ANSWER

The diagram to the right shows the relative energies of the atomic p orbitals, the resulting π molecular orbitals and the electron configuration.

(The same rules for filling orbitals applies to molecular orbitals as is used for atomic orbitals: review?)

The two electrons from the atomic p orbitals are now paired in the stabilised π bonding orbital. This is the highest occupied molecular orbital or HOMO in ethene (or any simple alkene).

In contrast, the π* anti-bonding orbital contains no electrons. It is the lowest unoccupied orbital or LUMO in ethene (or any simple alkene).