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I recently asked about how to distinguish between transition types. I got an answer there about determining the character or type of the orbitals.

My problem now is that I'm not sure how to determine the type of an orbital. How can you distinguish, either by inspection or some algorithm, whether an orbital is $\pi$, $\pi^*$ or $n$ on a system that contains double bonds and heteroatoms? For example what are the types of the HOMO and LUMO shown below.

LUMO orbital enter image description here

HOMO orbitalHomo

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  • $\begingroup$ In my opinion, distinction based on appearance may make only sense if you can find the pairs of bonding - bonding orbitals. In low symmetry / large molecules those are often not possible and not very informative either. $\endgroup$
    – Greg
    Jun 18, 2022 at 5:15

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Both orbitals look like $\pi$ orbitals, as both have nodes at the plane of the molecule, which is characteristic for $\pi$ systems. Occupied valence orbitals are usually considered to be bonding while excited unoccupied orbitals are often antibonding. In this particular case I woudn't say that the distinction between bonding and antibonding is 100% clear, but I'd call the LUMO $\pi^*$ in this case since the nodal structure can clearly be seen. I would also call this a charge transfer transition since you have a clear transfer of density from the left benzodioxole group to the central indene(?) group and the right benzene group.

But orbital characterization is not clear cut process. As you know yourself, there is no clear definition of these terms except in the most basic systems and I would recommend not to overthink the characterization. Just make sure that you you don't go against already established characterizations in literature.

Remark:

I would have preferred to post this as a comment as I don't provide a general methodology to solve the characterisation problem. But my comment didn't fit in the comments field.

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  • $\begingroup$ Hans Wurst thank you for your answer but by NBO program I can get the composition of my HOMO for exemple and know the percentage of my transition types by knowing the contribution of Lone pair and bonding orbital $\endgroup$
    – AIme999
    Jun 18, 2022 at 14:35
  • $\begingroup$ @ImeneYd I am aware of natural bonding and natural transition orbital schemes but never had to use them so I lack practical experience with them. You might consider posting an answer how you solved the problem if you found a good solution. $\endgroup$
    – Hans Wurst
    Jun 18, 2022 at 15:35
  • $\begingroup$ I didn't find the answer yet but I'm working on it thank you so much $\endgroup$
    – AIme999
    Jun 18, 2022 at 22:13

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