In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. HOMO and LUMO are sometimes collectively called the frontier orbitals, such as in the frontier molecular orbital theory.
The energy difference between the HOMO and LUMO is the HOMO–LUMO gap. Its size can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution. As a rule of thumb, the larger a compound's HOMO-LUMO gap, the more stable the compound.
The HOMO level is to organic semiconductors roughly what the maximum valence band is to inorganic semiconductors and quantum dots. The same analogy can be made between the LUMO level and the conduction band minimum.
In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur.
Subadjacent orbitals: NHOMO and SLUMO
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 named NHOMO for next-to-highest occupied molecular orbital and SLUMO for second lowest unoccupied molecular orbital. These are also commonly referred to as HOMO−1 and LUMO+1 respectively.
- Diels–Alder reaction
- Electron configuration
- Klopman-Salem equation
- Koopmans' theorem
- Woodward–Hoffmann rules
- Griffith, J. S. and L. E. Orgel. "Ligand Field Theory". Q. Rev. Chem. Soc. 1957, 11, 381–383.
- Bredas, J,-L. "Mind the gap!". Mater. Horiz. 2014, 1, 17–19.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "SOMO". doi:10.1351/goldbook.S05765.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "subjacent orbital". doi:10.1351/goldbook.S06067.
- "HOMO (Molecular Orbital) – an overview". ScienceDirect Topics.
- Fleming, Ian (21 December 2009). Molecular Orbitals and Organic Chemical Reactions. pp. 1–360. ISBN 9780470746592.
- OrbiMol Molecular orbital database