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I was taught that the only transition that occur in UV-vis spectroscopy are:

  • σ→σ*
  • π→π*
  • n→π*
  • n→σ*

Wikipedia seems to support this claim but I have trouble understanding why this must be the case. While I can understand that due to the large energy gap between σ and π* they might occur on rarer occasions I don't see why such a transition isn't possible. I did find some resources[1] that in fact say that this transition is possible and occurs in carbonyl but they didn't give any examples of it.

I also found an old research paper[2] that claims that this sort of transition does occur in some organoboranes where the HOMO is a σ orbital and LUMO is a π* orbital. This seems to be similar to carbon monoxide where σ is the HOMO instead of π but I wasn't able to find any resources that give a similar spectra for carbon monoxide but I do think that there isn't any selection rules that would prevent it as far as I know.

[1] Donald L. Pavia, Gary M. Lampman, George S. Kriz, James A. Vyvyan, Introduction to Spectroscopy (2008) p. 578

[2] Armstrong, D.R., Perkins, P.G. Calculation of σ→π* and π→σ* transitions in vinylboranes. Theoret. Chim. Acta 9, 412–416 (1968). https://doi.org/10.1007/BF00526401

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    $\begingroup$ In principle yes. It just depends on the relative energies of the levels, which may or may not lead to transitions in the UV - if being in UV really matters that is. Of course there will also be symmetry and spin selection rules to obey in any particular molecule. $\endgroup$
    – porphyrin
    Commented Apr 3 at 14:58

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