Forensic Chemistry – Classification Of UV Absorption Band

UV absorption bands can be broadly classified into three categories based on the nature of the electronic transition that gives rise to the absorption:

1. π-π* transitions: These transitions involve the excitation of an electron from a non-bonding π orbital to a π* antibonding orbital. These transitions typically occur in compounds containing conjugated systems of double or triple bonds, such as alkenes, alkynes, and aromatic compounds. The absorption bands associated with π-π* transitions are usually found in the range of 160-250 nm.
2. n-π* transitions: These transitions involve the excitation of an electron from a non-bonding n orbital to a π* antibonding orbital. These transitions typically occur in compounds containing heteroatoms such as oxygen, nitrogen, or sulfur, which have lone pairs of electrons that can be involved in the transition. The absorption bands associated with n-π* transitions are usually found in the range of 180-300 nm.
3. σ-σ* transitions: These transitions involve the excitation of an electron from a non-bonding σ orbital to a σ* antibonding orbital. These transitions typically occur in compounds containing saturated carbon-carbon or carbon-heteroatom single bonds, such as alkanes and ethers. The absorption bands associated with σ-σ* transitions are usually found in the range of 140-190 nm. However, these transitions are generally weaker than π-π* and n-π* transitions and are often not observed in UV spectra.