Interaction Vector. A New Concept to Approach the Intensity of the Secondary Transition of the Benzene Chromophore

Abstract

The new vector model (NVM),¹ designed for approaching the intensity of the secondary transition of the substituted benzene chromophore (towards 260 nm), has been based on MNDO calculations and on the SKLAR's vector model.^2–9 It leads to the simple relationship: ∊_sm,c = 4905 (TS + V). Experimental intensity is given as ∊_sm, the maximum of the smoothed absorption curve, as it has been defined by BALLESTER and RIERA¹⁰ (the calculated value is: ∊_sm,c). The vibrational component of intensity (V) is based, too, on their work. S is an increasing function of the number of substituents and of their ability to capture photons. It can be related to the UV cross section of the molecule, increasing when the UV cross section increases. T is the modulus of a vector related to the electronic transition moment. It is based on the distorsion of the π electronic charges, from a pure D_6h symmetry scheme. The MNDO method is used to calculate these charges. Thus, the modulus of the electronic transition moment being related to: μ = (TS)^1/2, is the result of two complementary approaches.

The basic difference between the new vector model and the SKLAR's one lies in the fact that the interactions between the substituents are taken into account in the ground state of the transition by the MNDO calculations. On the contrary, the main weakness in the SKLAR's vector model - which is sometimes erroneous by 50 or 100% - is in assuming that the vectors, related to the substituents, and composing the transition moment vector, are additive, independently of possible interactions among the substituents. In the present work we should like to consider these interactions, in order to systematize the vector process, avoiding the step of the MNDO calculations.