UV Absorption Band Classification
Based on the type of electronic transition that causes the absorption, UV absorption bands can be broadly divided into four categories:
- π-π*
- n-π*
- σ-σ*
- n-σ*
π-π* transitions: In these transitions, an electron is excited from a non-bonding orbital and moved into an antibonding orbital. Alkenes, alkynes, and aromatic compounds—compounds with conjugated systems of double or triple bonds—typically experience these transitions. The wavelengths between 160 and 250 nm are often where the absorption bands connected to π-π* transitions can be detected.
n-π*transitions: These transitions take place when an electron is excited from a n orbital that does not form bonds to a π* antibonding orbital. Since these transitions can involve lone pairs of electrons, they frequently happen in compounds containing heteroatoms like oxygen, nitrogen, or sulphur. The wavelengths between 180 and 300 nm are often where the absorption bands connected to n-π*transitions can be detected.
σ-σ* transitions: In these transitions, an electron is excited from a non-bonding orbital and moved into an antibonding orbital. Alkanes and ethers are examples of compounds with these transitions most frequently occurring in saturated carbon-carbon or carbon-heteroatom single bonds. The wavelengths between 140 and 190 nm are often where the absorption bands connected to σ-σ* transitions can be detected. These transitions, though, are often weaker than the a n-π*and π-π* transitions and are frequently missed in UV spectra.
n-σ*transition: In these transition, an electron is excited from an orbital that does not form bond to antibonding σ* orbital orbital.These transition are considered as the most weakest transition of all the types.
