- An electron-donating group adds more electrons to the system and thus increases electron-electron repulsion (or decreases the effective nuclear charge). As a consequence the HOMO and LUMO energies increase.
- An electron-withdrawing group removes electrons and, thus decreases the HOMO and LUMO energies.
- An electron-donating group usually acts through an occupied non-bonding orbital. This is energetically close to the HOMO. Therefore, it has a stronger effect on the HOMO than on the LUMO (at least in organic molecules).
- An electron-withdrawing group acts through a virtual orbital, which interacts more strongly with the LUMO.
- As a consequence, electron-donating and withdrawing groups are both expected to lower the HOMO-LUMO gap in organic molecules.
- Things are different for transition metal complexes. For example an electron-withdrawing fluorine group still lowers orbital energies. But it can affect the HOMO more strongly and increase the overall gap in fluorinated iridium complexes, see this Ref.
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3 comments:
Dear Felix,
What do you exactly mean by not believing in the existence of HOMOs and LUMOs? Are you saying that they simply don't exist? Or are you saying that they are physically meaningless concepts?
From photochemical standpoint, I vaguely understand that transitions are not necessarily always about HOMOs and LUMOs or any MOs, and natural transition orbitals sounds more physically meaningful to me. Or there are probably even better methods.
Also, the way I understand is that where exactly the HOMO and LUMO are distributed among the molecule also affect how strong the substituents affect their energies. For example, if you have a fluorine group on a carbon where HOMO is distributed, but not the LUMO, then LUMO will not be lowered in energy as effective as it would on HOMO. Perhaps, in the specific case of Ir-complexes, this effect exceeded in strength over your mentioned mechanism in how electron-withdrawing group is supposed to lower LUMO more than HOMO.
But of I am not a theoretical chemist, so I am more than likely making a very stupid mistake. What do you think?
Thank you,
Shun
Hi Shun, sorry for the late reply. The argument is that the HOMO and LUMO are just intermediate quantities in an approximate theory. There is no physical process that directly corresponds to the HOMO or LUMO. But of course, they are a useful model in many cases.
-Felix
>An electron-donating group usually acts through an occupied non-bonding orbital. This is energetically close to the HOMO. Therefore, it has a stronger effect on the HOMO than on the LUMO (at least in organic molecules).
>An electron-withdrawing group acts through a virtual orbital, which interacts more strongly with the LUMO.
It depends a bit on the chemical system. However, in most organic molecules your description is true when talking about mesomeric effects (+M and -M effects). However, inductive effects (+I, -I) usually 'push' or 'pull' electron density through the bonding orbitals (i.e. sigma bonds).
Also, depending on where in the molecule you attach your donating/withdrawing groups they may affect the HOMO and LUMO differently and in accordance with the molecule's mesomeric structure.
It's tends to get complex really fast even for the organic chemist and that's the reason why there's so many different models around to describe bonding. :)
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