Effects of Substituents on Ultra-violet Absorption Spectra

Abstract

SUBSTITUENTS in a conjugated system do not always shift the absorption bands of the system in the one direction. If we consider only the absorption band of lowest frequency, then the most commonly observed effect is a shift to still lower frequencies¹. However, Plattner and his co-workers² found that alkyl groups sometimes shift the lowest-frequency band of azulene in the opposite direction. Brown and Lahey³ found a similar alternating effect of alkoxy groups in N-methylaeridone. They emphasized that these shifts could be correlated with the π-electron densities of azulene⁴ and N-methylacridone⁵ in accordance with the following rule: substituents attached to a position of (local) low π-electron density increase the frequency of the first absorption band, while in positions of high π-electron density the frequency is decreased. It is significant that substituents in alternant⁶ hydrocarbons, such as naphthalene, do not occasion these alternating effects¹, and it is just for these systems that the molecular orbital approximation indicates a uniform electron density around the ring system⁶.