Energy gap between orbitals and stability

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Discussion Overview

The discussion revolves around the relationship between energy gaps in molecular orbitals, particularly in the context of conjugated double bonds, and how these gaps relate to the stability of such molecules. Participants explore theoretical aspects of bonding, stability, and the implications of molecular orbital theory, with references to spectroscopic techniques.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that a lower energy gap between bonding and antibonding orbitals leads to instability in molecules.
  • Others argue that in conjugated double bonds, the overlapping of wavefunctions results in an increase in the energy gap, contradicting the initial claim.
  • One participant expresses confidence in the idea that conjugation leads to a decrease in the energy gap, citing UV-Visible spectroscopy as evidence.
  • Another participant questions the meaning of "more stable" and suggests that stability can be assessed in various ways, including total energy and resistance to reactions.
  • A later reply introduces the concept of total pi electron energy and the specific molecular orbitals formed in conjugated systems, indicating that the HOMO-LUMO gap is not the sole factor in determining stability.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the energy gap increases or decreases with conjugation, and the discussion remains unresolved regarding the implications for molecular stability.

Contextual Notes

Participants express uncertainty about the definitions of stability and the specific conditions under which energy gaps are evaluated. There are references to spectroscopic methods and the behavior of pi electrons in different molecular structures, but these points are not fully resolved.

Tahmeed
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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.
 
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Tahmeed said:
However, in a conjugated double bond, the overlapping of wavefunctions cause the energy gap between the bonding and antibonding orbital go down.
No, this is not true. With conjugation, the energy gap increases.
 
DrDu said:
No, this is not true. With conjugation, the energy gap increases.
But i am pretty sure about it. This is why UV Visible spectometry works. You can check wikipedia.
 
Tahmeed said:
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.
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.
DrDu said:
No, this is not true. With conjugation, the energy gap increases.
In some very minor cases (that I don't remember), you are right. But generally, with more conjugation, the energy gap decreases.
 
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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