16.10 : UV-Vis 分光法: ウッドワード・フィーザー則

Conjugated π systems absorb in the UV–visible region where the wavelength of maximum absorbance, λ_max, increases with the extent of conjugation.

Woodward and Fieser devised a set of rules to predict λ_max based on the structure of conjugated systems.

Accordingly, λ_max is expressed as the sum of a base value and substituent contributions.

The base value is a fixed absorption value specific for acyclic, homoannular, and heteroannular dienes.

Substituent effects include contributions from extended conjugated double bonds, exocyclic double bonds, ring residues, and various substituents directly connected to the π system.

Let's work out an example.

Label the rings as A, B, and C. Since the two double bonds are part of the same ring, this is a homoannular diene with a base value of 253 nm.

There are three ring residues, each contributing 5 nm.

Additionally, there is one exocyclic double bond relative to ring B and an alkyl group directly attached to the π system. Both contribute 5 nm.

So, the calculated value of λ_max is 278 nm, close to the observed value of 275 nm.