I'm not great at most language rules, but I believe it still should be effected. Am I looking at this too rigid? As in, I am looking at it that the HOMO should have two electrons and no more, but in fact the electron density around the molecule is more fluid like? Why is the density wanting to move higher in energy to the HOMO? Are the orbitals from the electron donating group that are contributing to the MOs initially higher in energy, so that when they combine the energy difference between the substituent-based MO is closer to the HOMO, making it easier for the electrons to get enough energy to enter the HOMO?Īs for the affected vs. Technically the HOMO should already be at max capacity, and therefore should not take any more electron density. That is why I can imagine the LUMO should increase. I get that if you added an electron, then the orbital it goes into should increase. Remove electrons = lower energy orbitals. Tl dr Add electrons = higher energy orbitals. An electron-donating substituent usually has a similar effect on the energy levels. The energy of the electrons in every orbitals will be raised as there are only so much protons to attract them. LUMO : Ethylene > Nitroethylene (nitro = EWG)Īnother way to picture it is to analyze what would happen if you added an electron to your molecule. HOMO : Methoxyethylene (methoxy = EDG) > Ethylene At the same time, adding electron withdrawing substituents to a molecule will lower the energy of the LUMO as it is now easier to add electrons to the said molecule.Īs a very simple example, let's take ethylene: The reason is quite simple, if you add electron density to a molecule, it becomes easier to remove electrons from the said molecule, ie. So why would adding electron density to the backbone of the molecule cause the HOMO to increase?Ī simple rule which is pretty much always respected is that adding electron-rich groups to a molecule will raise its HOMO. I can see how the LUMO can become destabilized because it has room for electrons to be added. In these systems, the HOMO which is sigma bonding should already contain 2 electrons. What I don't understand is why the HOMO should be effected by an electron donating group. What I know is that when an inductively donating group is added to the group 14 element, both the HOMO and LUMO becomes destabilized. This could be explained in other systems, but that is what I'm looking at. I am looking at oligomeric organometallic chains of heavy group 14 elements. We no longer consider energy levels, but energy bands.Alright, so I am currently working on trying to understand how various substituents effect the overall HOMO and LUMO of a molecule in an attempt to reason why some weird stuff is going on in my research, and I could really use someone to explain this to me because somewhere along the way I missed something important or didn't connect some dots. in an aggregate), there are so many sublevels that we no longer perceive their discrete nature: they form a continuum. When there are enough molecules influencing each other (e.g. There are as many vibrational sublevels as there are molecules that interact together.

This splitting produces vibrational sublevels which each have their own energy, slightly different from one another. When the molecule forms a dimer or an aggregate, the proximity of the orbitals of the different molecules induce a splitting of the HOMO and LUMO energy levels. The energy difference between the HOMO and LUMO level is regarded as band gap energy. The same analogy exists between the LUMO level and the conduction band. The HOMO level is to organic semiconductors what the valence band is to inorganic semiconductors.

Homo vs lumo