A New Approach to the Intensity of the Secondary Transition of the Benzene Chromophore with-CH₃,-OH,-OCH₃, Substituents

Abstract

Since the beginning of the UV absorption spectroscopy, the intensity of the secondary transition of the benzene chromophore in organic derivatives has been a puzzling challenge. The well known vector model has led to some qualitative evaluation of intensity, but accuracy is very low, and sometimes leads to no less than 50% or 100% error. ^1–8 It is designed for very weak perturbations only, as in methylbenzenes. But, even in that latter case, it is not completely satisfactory. Nevertheless, that model is very easy to use; it allows to determine the sensitive positions around the benzene nucleus: those which could enhance or quench intensity for a given pattern of substitution. On that ground it has been of much help for qualitative evaluations. With SCF methods and CI calculations, evaluation of transition moments for several benzene derivatives has been possible on a firmer ground. In fact, such an approach is useful mainly from a theoretical point of view, when accuracy is not the first aim, for some choosen molecules at the same time, since the involvement of computation is far from being negligible, and since the method is not part of the ones that are of common use in organic chemistry ^9,10 (for a recent example on oscillator strength calculations of π systems see:¹¹). In other words, it is not perfectly valuable on a large scale and routine approach and, from that point of view, UV spectroscopy of benzene derivatives appeared sometimes, from 1965 till now, as being in a cul-de-sac, although it was under thorough examination (for example: 12–30). Until recently ab initio calculations have failed to reach a fit with experiment better than 1 eV for the spectrum of the benzene molecule^31–32)