Larger molecules have smaller HOMO-LUMO Gaps?

[Hyo X]

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.

 

[DrDu]

If it were true in general, there wouldn't be any insulators.

 

[tadchem]

When it comes to chemistry, in general there are no reliable generalities.

 

[DrDu]

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.

 

[LudusRex]

The statement that larger molecules have smaller HOMO-LUMO gaps is not entirely true. While it is generally observed that longer saturated molecules tend to have smaller HOMO-LUMO gaps compared to shorter saturated molecules, the same cannot be said for conjugated molecules. Conjugated molecules, which have alternating single and double bonds, tend to have larger HOMO-LUMO gaps compared to their saturated counterparts.

This is because in conjugated molecules, the delocalization of electrons along the pi bonds extends the HOMO and LUMO orbitals, resulting in a larger gap between them. On the other hand, in saturated molecules, the electrons are localized on individual atoms, leading to a smaller HOMO-LUMO gap.

Additionally, the size of a molecule does not solely determine the HOMO-LUMO gap. Other factors such as the structure, functional groups, and electronic properties of the molecule also play a significant role. For example, a branched molecule may have a smaller HOMO-LUMO gap compared to a linear molecule of similar size due to steric hindrance and the influence of neighboring groups on the electronic structure.

Therefore, it is not accurate to make a general statement that larger molecules always have smaller HOMO-LUMO gaps. The relationship between the size of a molecule and its HOMO-LUMO gap is complex and can vary depending on the specific molecule and its structural and electronic characteristics.