Near UV Spectra of Phenoxyacetic Acid and Phenoxypropanoic Acid

Abstract

The secondary transition of the benzene molecule lies towards 250-260 nm. It is electronically forbidden because of the D_6h symmetry of the molecule. The intensity of the transition is low (?_max = 220; medium : cyclohexane) and very sensitive to interactions with the surrounding parts of the molecule likely to perturb the symmetry. Substituents distort all the more the symmetry as their orbitals couple with the orbitals of the x system. They increase the intensity of the transition according to a vector scheme taking into account the positions of the substituents around the nucleus, and the strength of their coupling (SKLAR). ^7-14It has been shown that the too much qualitative SKLAR's approach could be superseded by the much more quantitative and more accurate NVM (New Vector Model) l5 grounded on MNDO calculations, or the IVM (Interaction Vector Model) using a more empiric approach.