Identification of HOMO/LUMO in radicals

In summary, the conversation discusses the identification of HOMO and LUMO orbitals in radical molecules, specifically focusing on molecules with singly occupied orbitals. The majority of cases, such as O2, have two singly occupied antibonding orbitals that are considered the HOMO, with the antibonding sigma*2s being the LUMO. However, in molecules like NO where only one antibonding orbital is singly occupied, there is confusion on which orbital is considered the HOMO and LUMO. The conversation also mentions the asymmetry of energy levels in non-homoatomic bi-atomic radicals and how it can affect the filling-up of orbitals. The cited reference confirms the correctness of the energy diagrams
  • #1
Manganese
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TL;DR Summary
How to identify HOMO/LUMO in molecules with singly occupied orbitals?
Hello,
as the title says, how do you treat singly occupied orbitals in radical molecules, when trying to identify the HOMO and LUMOs?

In the majority of cases I stumbled upon, like O2, both the antibonding orbitals, pi*2px pi*2py, are singly occupied, so they would be considered the HOMO, with the antibonding sigma*2s being the LUMO. Same goes for other radicals that have one fully occupied orbital (eg pi*2px) and one singly occupied (eg pi*2py).

But in molecules like NO, where only one of the two antibonding orbitals is singly occupied (eg, the pi*2px OR the pi*2py), what is considered the HOMO and which the LUMO? Are both the pi*2px and pi*2py considered HOMO even if only one of them is occupied (and only by 1 e), with the sigma*2s being the LUMO? Or both the pi*2px and pi*2py are considered HOMO and LUMO at the same time (is that even possible?) Even if we call the singly occupied orbitals as SOMO, what would be the corresponding LUMO?
 
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  • #3
Hyperfine said:
Molecular Orbital Diagram of NO
That is for the ground state.
Are the energy diagrams correct in the cited reference? For non-homoatomic bi-atomic radicals, the energy levels of the orbitals of the heavier atom are drawn downwards. That asymmetry is characteristic of such radicals. Whether that affects the filling-up of the orbitals, thus defining which is LUMO and which is HOMO, depends on the atoms involved. So, they are nearly leveled for NO, but quite prominent for HF.
 
  • #5
Hyperfine said:
Yes. Also see: What is the electron configuration of nitrogen monoxide?
Sorry to press on, but while this latter configuration agrees with my description, the former doesn't. In the first citation, no attention is paid to the difference in energy levels for the corresponding orbitals for N and O. It's obvious unless the text explains sth about it. I wouldn't like to further comment on that.
 
  • #6
I did not say that the diagrams were to scale. They present the molecular orbitals of the molecule, and that was the question originally posed.
 
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  • #7
Hyperfine said:
I did not say that the diagrams were to scale. They present the molecular orbitals of the molecule, and that was the question originally posed.
Sorry for the late reply, but my question was not how to draw the MO of NO. I wanted to know which one would be identified as HOMO and which as LUMO, considering that there's only 1 electron on the highest level but there's also a free orbital next to it
 
  • #8
In radicals, you typically speak of SOMOs, single occupied molecular orbitals in contrast to the empty LUMOs and doubly occupied HOMOs.
 

FAQ: Identification of HOMO/LUMO in radicals

What are HOMO and LUMO in the context of radicals?

HOMO stands for Highest Occupied Molecular Orbital, while LUMO stands for Lowest Unoccupied Molecular Orbital. In the context of radicals, which are species with unpaired electrons, the identification of HOMO and LUMO helps in understanding their reactivity and stability. The HOMO is typically involved in donating electrons, while the LUMO is involved in accepting electrons during chemical reactions.

How can I identify the HOMO and LUMO in a radical?

The identification of HOMO and LUMO in a radical can be done using computational chemistry methods, such as Density Functional Theory (DFT) or Hartree-Fock calculations. These methods allow for the calculation of molecular orbitals, from which the energies and occupancy of HOMO and LUMO can be determined. Visualizing these orbitals can also provide insights into their shapes and interactions.

What is the significance of HOMO-LUMO gap in radicals?

The HOMO-LUMO gap is a crucial parameter that indicates the energy difference between the HOMO and LUMO. In radicals, a smaller HOMO-LUMO gap often signifies higher reactivity, as it suggests that the radical can easily participate in electron transfer processes. Conversely, a larger gap may indicate greater stability and lower reactivity.

Can the HOMO and LUMO of a radical be experimentally determined?

While the HOMO and LUMO can be theoretically calculated, experimental techniques such as UV-Vis spectroscopy can provide indirect information about these orbitals. By analyzing the absorption spectra, one can infer the energy levels of the HOMO and LUMO based on the wavelengths of light absorbed during electronic transitions.

How do substituents affect the HOMO and LUMO in radicals?

Substituents on a radical can significantly influence the energies of the HOMO and LUMO. Electron-donating groups tend to raise the energy of the HOMO, making the radical more reactive, while electron-withdrawing groups lower the energy of the HOMO, potentially stabilizing the radical. Similarly, these substituents can also affect the LUMO energy, thereby altering the HOMO-LUMO gap and the overall reactivity of the radical.

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