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Energy gap between orbitals and stability

  1. Jun 8, 2017 #1
    We know that the lower the energy gap between bonding and antibonding orbital the easier it is to break the bond making it unstable. However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down. Then why conjugated double bonded molecules are more stable??
    I aint sure whether this should go to chemistry part of the forum or it is fine to post here. so sorry in advance if this is in wrong part.
     
  2. jcsd
  3. Jun 9, 2017 #2

    DrDu

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    Science Advisor

    No, this is not true. With conjugation, the energy gap increases.
     
  4. Jun 10, 2017 #3
    But i am pretty sure about it. This is why UV Visible spectometry works. You can check wikipedia.
     
  5. Jun 10, 2017 #4

    HAYAO

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    I think this is the right section of the forum. It would still be fine if it was in general physics section in my opinion.

    I don't think I understand you well, but what do you mean "more stable"? There are several ways you can consider a molecule stable such as entire energy of the system, inertness to photo-oxidization or reduction, decomposition point, etc.


    In some very minor cases (that I don't remember), you are right. But generally, with more conjugation, the energy gap decreases.
     
  6. Jun 12, 2017 #5
    It may be the case that the HOMO-LUMO gap decreases, but that is not the whole story. You have to think about the total pi electron energy. What MOs are produced by the overlap of the 6 pi orbitals in benzene, for example, or the four pi orbitals in butadiene? How are they filled? What is the pi electron energy compared to that of forming three (or two) separate pi bonds?
     
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