Question

What is the HOMO-LUMO gap and how does it change the properties of a substance?

Answer 1

HOMO-LUMO gap:

• “HOMO” stands for Highest occupied molecular orbital while “LUMO” stands for Least unoccupied molecular orbital.
• The HOMO is the highest energy molecular orbital that consists of electrons while the LUMO is the next highest energy orbital that is empty.
• The LUMO is the lowest energy place to excite an electron.
• The energy difference between the HOMO and LUMO or HOMO-LUMO gap is the lowest energy excitation of electrons that exists in a molecule.

HOMO-LUMO gap changing the properties of a substance:

• The excitations become easier as the HOMO-LUMO gap converges which is responsible for the color in transition metal complexes.
• The greater the mobility of the $\mathrm{\pi }$ electrons in large conjugated $\mathrm{\pi }$ orbital systems, the greater the distribution of the energy throughout the molecule stabilizing it.
• Hence, smaller HOMO-LUMO gaps correspond to better stability.