Larger molecules have smaller HOMO-LUMO Gaps?

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Discussion Overview

The discussion centers on the relationship between molecular size and the HOMO-LUMO gap, specifically comparing long versus short saturated and conjugated molecules. Participants explore whether larger molecules generally exhibit smaller gaps and the implications of this relationship in different molecular contexts.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the generality of the claim that larger molecules have smaller HOMO-LUMO gaps, suggesting that if it were universally true, it would imply the existence of no insulators.
  • Another participant argues that in chemistry, reliable generalities are often elusive, indicating skepticism about broad claims.
  • A participant notes that in saturated molecules, the HOMO-LUMO gap is largely independent of system size, while in conjugated systems, the gap tends to decrease with increasing size. However, they also mention that larger conjugated molecules may experience new gaps due to Peierls instability.

Areas of Agreement / Disagreement

Participants express disagreement regarding the generality of the relationship between molecular size and HOMO-LUMO gaps, with no consensus reached on the validity of the initial claim.

Contextual Notes

Participants highlight the complexity of the relationship, noting that different types of molecules (saturated vs. conjugated) may behave differently regarding their HOMO-LUMO gaps, and that assumptions about size and stability may vary.

Hyo X
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Is it true that in general, larger molecules will have smaller HOMO-LUMO gaps?

In this case consider that we are comparing long vs short saturated molecules,
and long vs. short conjugated molecules.
 
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If it were true in general, there wouldn't be any insulators.
 
When it comes to chemistry, in general there are no reliable generalities.
 
In saturated molecules, the gap will hardly depend on system size while in conjugated systems, it will decrease with system size. However, in larger conjugated molecules, new gaps may open due to the Peierls instability.
 

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