What is the significance of the absolute HOMO in spectroscopy?

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Discussion Overview

The discussion centers on the concept of the absolute highest occupied molecular orbital (HOMO) in spectroscopy, exploring its definition, measurement techniques, and implications in molecular energy levels. Participants consider the relationship between HOMO, lowest unoccupied molecular orbital (LUMO), and various spectroscopic methods, particularly UV-Vis and cyclic voltammetry.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the absolute HOMO refers to the highest occupied molecular orbital in a molecule, but express uncertainty about how to determine it using spectroscopy.
  • There is a suggestion that the term "absolute" may imply a measurement of absolute energy, though this is not universally accepted.
  • One participant argues that UV-Vis spectroscopy cannot directly determine HOMO or LUMO energies, as it measures transition energy, which may not correspond to the HOMO-LUMO gap.
  • Another participant emphasizes that the lowest energy observed in UV-Vis may not necessarily indicate a transition from HOMO to LUMO, as it could involve other molecular orbitals.
  • Some participants mention cyclic voltammetry as a potential method for determining the HOMO-LUMO gap, contingent on the stability of the molecule in question.
  • Koopman's theorem is referenced, suggesting that the ionization energy can provide an approximation for the energy of the HOMO, with photoemission spectroscopy proposed as a measurement technique.
  • Clarifications are made regarding the mixing of transitions in UV-Vis spectra, indicating that transitions may involve contributions from multiple molecular orbitals.

Areas of Agreement / Disagreement

Participants express a range of views on the definition and measurement of the absolute HOMO, with no consensus reached on the best approach or the implications of the term "absolute." The discussion remains unresolved regarding the most effective methods for determining HOMO and LUMO energies.

Contextual Notes

Limitations include the dependence on the definitions of "absolute" and "relative" energy levels, as well as the unresolved nature of how different spectroscopic techniques relate to HOMO and LUMO measurements.

Chemmjr18
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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.
 
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I've never heard of it Perhaps it is talking about the absolute energy of HOMO?
 
HAYAO said:
I've never heard of it Perhaps it is talking about the absolute energy of HOMO?
Yes. I think it means the absolute energy...
 
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".
 
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HAYAO said:
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!
 
One thing I must tell you that might have been misleading.

HAYAO said:
...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 B2u excited states.
 

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