3.01 | The MO-Diagram of π-Conjugated Systems

Friday May 19th 2023 | In this online lesson, we take our knowledge on how to construct the MO-diagram of complex main group elements to organic compounds that have π-Conjugated Systems. We start with the most simple example, ethylene, which is also the first example in which there is no central atom. We construct the MO-Diagram by grouping the carbon atoms and hydrogen atoms separately and determining their symmetry adapted linear combinations. By doing so we realize that for the two carbon atoms, the MO-diagram of C2 fragment is generated including the bonding and anti-bonding interaction. Having established the symmetry adapted linear combinations, we constructed the MO-diagram of ethylene and arranged the molecular orbitals (energy wise) according to the number of nodes we observe. Now that the MO-diagram of ethylene is established, we move forward to the MO-diagram of butadiene. When we establish the irreducible representations of the four carbon atoms, we find that the Pz orbitals -that form the π-conjugated system – are always different, which turns out to be a general rule. It also turns out that these orbitals happen to be the HOMO and the LUMO and that by understanding their symmetry we can understand their reactivity. Finally, this lesson concludes with analyzing the HOMO/LUMO orbitals of the cyclopentadienide ligand (CP) that is commonly encountered in organometallic chemistry.

The link to the video can be found below: