Inchemistry,HOMO andLUMO are types ofmolecular orbitals. The acronyms stand forhighest occupied molecular orbital andlowest unoccupied molecular orbital, respectively. HOMO and LUMO are sometimes collectively called thefrontier orbitals, such as in thefrontier molecular orbital theory.
The energy difference between the HOMO and LUMO isthe HOMO–LUMO gap. Its size can be used to predict the strength and stability oftransition metalcomplexes, as well as the colors they produce in solution.[1] As a rule of thumb, the smaller a compound's HOMO–LUMO gap, the less stable the compound.[2]
The HOMO level is toorganic semiconductors roughly what the maximumvalence band is to inorganicsemiconductors andquantum dots. The same analogy can be made between the LUMO level and theconduction band minimum.[3]
In organometallic chemistry, the size of the LUMO lobe can help predict whereaddition to pi ligands will occur.
ASOMO is asingly occupied molecular orbital such as half-filled HOMO of aradical.[4] This abbreviation may also be extended tosemi occupied molecular orbital.
If existent, the molecular orbitals at one energy level below the HOMO and one energy level above the LUMO are also found to play a role in frontier molecular orbital theory. They are namedNHOMO fornext-to-highest occupied molecular orbital andSLUMO forsecond lowest unoccupied molecular orbital.[5] These are also commonly referred to as HOMO−1 and LUMO+1 respectively.[6]
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