What kind of transition is allowed in UV for benzene ring?
Benzene aromatic ring archetype possesses planar conjugated system with six π electrons delocalized over three formal double bonds, satisfying Hückel rule. Ultraviolet absorption involves promotion of an electron from filled bonding π orbital to vacant antibonding π* orbital. This π→π* transition is symmetrically allowed, intense, with molar extinction about 200 to 10,000 M-1 cm-1 and characteristic vibronic fine structure near 255 nm known as primary and secondary benzenoid bands. σ→σ* requires far vacuum UV, n-related transitions absent due to lack of heteroatom lone pairs, explaining phenyl
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
