December 07, 2022, 02:03:28 AM
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Topic: MO energy levels for anion relative to neutral species, discrepancy in logic  (Read 1700 times)

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Offline spirochete

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I attached a picture where an MO digram for acetylide ion and acetone are compared. It says that the orbitals for acetylide are generally shifted up higher in energy because it is an anion.

But when I look up the energies for the pi systems of an allyl cation and an allyl anion, the relative energies of the orbitals are identical. The only difference is that the MO diagram for the allyl anion has more electrons in it. Even when I look it up in the oxford primers by Fleming that involve MO theory and pericyclic reactions, I can't find any mention of the orbitals having different energies based on whether the species is a cation or an anion or neutral.

How do I reconcile this information? If possible, without any math please.

Offline Babcock_Hall

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I am well outside of my comfort zone, but I am willing to hazard a guess.  There is a difference between relative energy levels and absolute energy levels.  Does that help?

Offline rolnor

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Just looking at the fact that anions an cations are generally very reactive compared to neutral molecules should give you a hint.

Offline Babcock_Hall

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Another way to think about this would be to find the reduction potential of the anion.

Offline spirochete

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I'm not sure how these things help because I seem to be able to come up with alternative explanations that are not explained by charge. I also don't understand why the lower level orbitals are raised up in energy for the case of acetylide relative to acetone (or acetylide relative to acetylene, also), but not when comparing the allyl anion to the allyl cation. Acetylide compared to acetylene, and acetylide compared to acetone, seem to contradict the comparison of allyl cation to allyl anion.

Acetylide ion has a non bonding orbital that acetylene lacks, and so it is going to have one orbital that is higher energy for sure.  It also makes sense that acetylide would have mostly higher energy orbitals than acetone, but that could be explained due to the lack of an electronegative oxygen, which raises up the energy of the orbitals for that reason.

Also, there is a separate thing that makes a chemical reaction go faster which I think is separate from MO energy levels: electrostatic attraction between negative charge on acetylide ion and partial positive carbonyl carbon.


Offline rolnor

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I think your calculations  are missing something very vital. Why should these two compounds/species be compared this way at all? Why should they be similar?

Offline spirochete

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I get the vague sense that I'm making some comparisons that are inappropriate, but I'm not sure why.

But if a species being an anion should raise all the orbital energies, and a species being a cation should lower all the orbital energies, specifically because of the charge, then why shouldn't this also be true when comparing allyl anion to allyl cation? I have tried looking up calculated energies for the orbitals but I have failed to locate them.

Offline rolnor

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A cation is also, generally,  much higher in energy. What lead you in to do this calculations? Do you understand what it means that a compound has high energy? Why do chemical reactions occur at all?

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