Limitations of the applicability of regional symmetry rules: The aromatic1 L b -cotton effect

Abstract

A critical examination concerning the validity of symmetry rules, which correlate theCD of the inherently achiral, chirally perturbed benzene chromophore in the1Lb wavelength region with the absolute configuration is presented. It is shown, that the derivation of generally applicable symmetry rules is interdicted by fundamental arguments as is deduced by studying the origin of optical activity in the above mentioned electronic transition. Further evidence is given, that the generation of parallel components of the electric and magnetic moment is due to a vibronic perturbation. While the electric transition moment can roughly be estimated using spectroscopic partial moments, the magnetic transition moment will depend on the properties of the ▪-skeleton including both conformational rigidity and the mass distribution (which latter involves the substitution pattern). Thus the uncertainity concerning the direction of the magnetic moment prevents the deduction of a symmetry rule applicable to different molecular frames. Moreover, the relevance of such rules is shown to be limited even within distinct frames. A method is developed to find the conditions which imply the establishment of an unequivocal sector rule for a definite substitution pattern. Moreover, this may afford a tool for a rough localization of the magnetic transition moment.