HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The difference of the energies of the HOMO and LUMO, termed the band gap, can sometimes serve as a measure of the excitability of the molecule: the smaller the energy, the more easily it will be excited.

The HOMO level is to organic semiconductors and quantum dots what the valence band is to inorganic semiconductors. The same analogy exists between the LUMO level and the conduction band. The energy difference between the HOMO and LUMO level is regarded as band gap energy.

When the molecule forms a dimer or an aggregate, the proximity of the orbitals of the different molecules induces a splitting of the HOMO and LUMO energy levels. This splitting produces vibrational sublevels which each have their own energy, slightly different from one another. There are as many vibrational sublevels as there are molecules that interact together. When there are enough molecules influencing each other (e.g. in an aggregate), the number of sublevels are large enough to be perceived as a continuum rather than discrete levels. We no longer consider energy levels, but energy bands. ^[1]

Contents 1 Identifying the HOMO and the LUMO 2 SOMO 3 See also 4 References

[edit] Identifying the HOMO and the LUMO

To identify the HOMO and the LUMO of a given molecule one can

1. Find out all the molecular orbitals and fill them with the available electrons, or
2. Use a generic ordering of orbitals, and use valence bonding for σ-type and lone pair orbitals, and molecular orbitals for π-systems as an approximation.

The generic ordering of molecular orbitals (from highest energy to lowest energy):

• σ* - almost never occupied in the ground state
• π* - very rarely occupied in the ground state
• n - nonbonding (ie. lone pairs)
• π - always occupied in compounds with multiple bonds (Pi Bond)
• σ - at least one occupied in all molecules (Sigma Bond)
• a - an empty orbital

The kinds of electrons present are identified through observation of the Lewis structure. Although this generic system will identify the correct HOMO and LUMO most of the time, there are exceptions to this classification. To identify any resonances in the system, the entire π system must be considered.

[edit] SOMO

A SOMO is a singly occupied molecular orbital and defined as half the HOMO of a free radical ^[2]. It shows the difference of energies between orbitals.

[edit] See also

• Diels-Alder reaction
• Electron configuration
• Koopmans' theorem
• Ligand
• Organic semiconductor

[edit] References