In
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 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
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 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 will occur.
SOMO
A ''SOMO'' is a ''singly occupied molecular orbital'' such as half-filled HOMO of a
radical. 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.
See also
*
Diels–Alder reaction
*
Electron configuration
In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon ato ...
*
Klopman–Salem equation
*
Koopmans' theorem
Koopmans' theorem states that in closed-shell Hartree–Fock theory (HF), the first ionization energy of a molecular system is equal to the negative of the orbital energy of the highest occupied molecular orbital (HOMO). This theorem is named afte ...
*
Ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
*
Woodward–Hoffmann rules
References
Further reading
*
External links
OrbiMolMolecular orbital database
Quantum chemistry
{{quantum-chemistry-stub