What is the significance of the absolute HOMO in spectroscopy?

[Chemmjr18]

What exactly is the absolute highest occupied molecular orbital? I think, as the name suggests, it's the absolute highest occupied molecular orbital in a molecule. Could it be determined using spectroscopy? Maybe, depending on the type. You could use UV-Vis if the energy gap between the HOMO and LUMO is in the UV range, but I'm not exactly sure how you'd do this (i.e., what model you'd use). Maybe from the max wavelength on the UV-Vis spectrum. On the other hand, the word "absolute" is throwing me off because I'm not sure that as scientist we can determine the absolute of anything! Sure, we can make some highly accurate approximations but absolute measurement?! I'm not so sure. I'm working on an assignment that asks about absolute energy levels. Any help is appreciated. By the way, I'm not looking for someone to give me an answer, just help guide my thinking.

 

[HAYAO]

I've never heard of it Perhaps it is talking about the absolute energy of HOMO?

 

[Chemmjr18]

I've never heard of it Perhaps it is talking about the absolute energy of HOMO?

Yes. I think it means the absolute energy...

 

[HAYAO]

You will not be able to determine HOMO, LUMO, or HOMO-LUMO gap energy from UV-vis. Absorption spectroscopy measures transition energy, not HOMO-LUMO energy although they are indeed closely related. However, there is no guarantee that the lowest energy observed in UV-vis corresponds to a transition from HOMO to LUMO. It could be mixed with transition between other MOs. Also transition energy includes Coulomb energy and exchange energy along with HOMO-LUMO gap.

If you want to determine HOMO-LUMO gap, you might want to try cyclicvoltammetry instead (if the molecule can survive). If the absolute energy of the redox potential of a reference electrode is known (for example Ferrocene), then in principle, you should be able to know the absolute energy of HOMO and LUMO as well.EDIT: Just so you know, the term "absolute" does not mean "completely accurate". It is a word used to mean the opposite of "relative".

 

[Chemmjr18]

You will not be able to determine HOMO, LUMO, or HOMO-LUMO gap energy from UV-vis. Absorption spectroscopy measures transition energy, not HOMO-LUMO energy although they are indeed closely related. However, there is no guarantee that the lowest energy observed in UV-vis corresponds to a transition from HOMO to LUMO. It could be mixed with transition between other MOs. Also transition energy includes Coulomb energy and exchange energy along with HOMO-LUMO gap.

If you want to determine HOMO-LUMO gap, you might want to try cyclicvoltammetry instead (if the molecule can survive). If the absolute energy of the redox potential of a reference electrode is known (for example Ferrocene), then in principle, you should be able to know the absolute energy of HOMO and LUMO as well.EDIT: Just so you know, the term "absolute" does not mean "completely accurate". It is a word used to mean the opposite of "relative".

Wow! This was a really great answer. Thanks!

 

[HAYAO]

One thing I must tell you that might have been misleading.

...there is no guarantee that the lowest energy observed in UV-vis corresponds to a transition from HOMO to LUMO. It could be mixed with transition between other MOs...

When I said "mixed" it does not mean that there are two absorption bands that are overlapped in the spectra. It literally means that the transition itself is mixed with another transition. This is because configuration interaction mixes states of same symmetry. For example, the lowest transition of Benzene has contribution from two separate B_2u excited states.

 

[DrDu]

According to Koopman's theorem
https://en.wikipedia.org/wiki/Koopmans'_theorem
the (negative of the )ionisation energy is equal to the energy of the HOMO, at least in good approximation.
The ionisation energy can be measured using photoemission spectroscopy
https://en.wikipedia.org/wiki/Photoemission_spectroscopy