Effects of substituents on the electronic absorption spectra of 3-N-(4-substituted phenyl)-5-carboxy uracils in different solvents. Part II.

Abstract

Absorption spectra of some 3-N-(4-substituted phenyl)-5-carboxy uracils, previously determined in twelve solvents, are correlated with substituent sp+ values or by dual substituent parameter correlations involving sl and sR+ values. The substituents at the phenyl nucleus are as follows: H, Cl, Br, I, Me, Et, OMe, COOH and NO2. Considering the structure of the investigated system, it is very probable that the effect of the substituent at the phenyl nucleus on the electronic spectra of the substituted uracils is influenced by resonance interactions between the electron pairs of both the nitrogen atoms, the carbonyl groups and the substituent itself. The polarity of the reaction medium also affects the complex resonance interactions in the molecules of the investigated compounds. The obtained results support a suggestion made by Brownlee and Topsom that UV/visible spectral shifts can conform to reasonable precise linear free energy relationships if the substituent is not a part of the chromophoric system.

Keywords

• absorption spectra
• substituent effects
• solvent effects
• intramolecular hydrogen bond