Basics of UV-visible absorption spectroscopy

The Woodward and Fieser rules

The empirical rules of Woodward-Fieser are aimed at predicting the wavelength of the maximum absorption in the UV-visible domain for organic compounds. They edict that some groups of functions based on their nature may induce or not a different increase of the wavelength for the maximum absorption: they are named chromophores. The wavelengths of maximum absorption for dienes (basic structures) as well as increments for several groups constitute the Woodward-Fieser rules. For polyenes, the Fieser-Kuhn rule is used instead.

Examples of 3 groups and their absorption maximum wavelength. Acyclic Diene: 217nm, Heteroannular: 214nm, Homoannular: 253nm.

List of different chromophores is the associated increment. Ester: +0nm, Alkyl: +5nm, exoclyclic double bond: +5nm, ether: +6nm, -Cl -Br -halogen: +10nm, sulfide: +30nm, conjugated double bond: +30nm, secondary amine: +60nm, phenyl: +60nm.

Fieser-Kuhn rule

λ_max = 114 + 5M + n (48,0 - 1,7 n) - 16,5 R_endo - 10 R_exo
where
λ_max is the wavelength of the maximum absorption
M is the number of alkyl substituents/ residues of cycle in the conjugated system
n is the number of conjugated double bonds
R_endo is the number of rings with endocyclic double bonds in the conjugated system
R_exo is the number of rings with exocyclic double bonds in the conjugated system
and ε_max = (1,74 x 104) n where ε_max is the maximum absorption coefficient.