chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, HOMO and LUMO are types of

molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding ...

s. 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 In chemistry, frontier molecular orbital theory is an application of molecular orbital theory describing HOMO and LUMO, HOMO–LUMO interactions. History In 1952, Kenichi Fukui published a paper in the ''Journal of Chemical Physics'' titled "A m ...

Gap

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 In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

es, as well as the colors they produce in solution. As a rule of thumb, the smaller a compound's HOMO–LUMO gap, the less stable the compound. Recent quantum‐chemical analyses of over 700 compounds demonstrated that terrestrial secondary metabolites exhibit HOMO–LUMO gaps on average about 2 eV narrower than organic molecules found in carbonaceous meteorites, and that combining gap width with hydrophilicity creates a robust discriminator between biotic and abiotic chemistries. This suggests that the HOMO–LUMO gap may serve as a useful biosignature in future life‐detection instruments, by highlighting aqueous, highly reactive compounds of biological origin.

Semiconductors

The HOMO level is to

organic semiconductors Organic semiconductors are solids whose building blocks are pi-bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such as nitrogen, sulfur and oxygen. They exist in the form of molecular crystals or ...

roughly what the maximum

valence band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...

is to inorganic

semiconductors A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping levels ...

and

quantum dot Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic i ...

s. The same analogy can be made between the LUMO level and the

conduction band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...

minimum.

Organometallic chemistry

In organometallic chemistry, the size of the LUMO lobe can help predict where

addition to pi ligands Addition (usually signified by the plus symbol, +) is one of the four basic operations of arithmetic, the other three being subtraction, multiplication, and division. The addition of two whole numbers results in the total or '' sum'' of th ...

will occur.

SOMO

A ''SOMO'' is a ''singly occupied molecular orbital'' such as half-filled HOMO of a

radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...

. This abbreviation may also be extended to ''semi occupied molecular orbital''.

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.

References

External links

OrbiMol
Molecular orbital database Quantum chemistry {{quantum-chemistry-stub

Gap