The Intensity of the Secondary Transition of the Benzene Chromophore with Fused Rings. The New Vector Model

Abstract

It has been shown recently ¹ that it is possible to calculate the intensity of the secondary transition (towards 260 nm) of the substituted benzene chromophore with better accuracy than it is possible with the simple Sklar vector model. ^2–9 This new approach is based on MNDO calculations. It leads to a simple relationship, which is applied - for the time being - to π-donating substituents such as -CH₃, -OH, -OCH₃, since we are mainly involved in the study of natural products:

ε_sm is the maximum of the smoothed absorption curve, as it has been defined by BALLESTER and RIERA. ₁₀ It avoids the problems linked to the vibrational fine structure which depend strongly on the relative importance of the two progressions of the secondary transition,₁₁ on the interactions of the molecule with the solvent,¹² etc. Our relationship has been tested for the values O < ε_sm < 3600, for molecules with weak steric strain (no fused rings).