Vibronic Coupling Dominates the Electronic Circular Dichroism of the Benzene Chromophore 1Lb band

The alkylbenzene derivatives (R)-PhCH(CH3)Bu-t (1) and (R)-PhCH-(CH3)Pr-i (2) were taken as paradigms of chiral benzene compounds and their vibronic electronic circular dichroism (ECD) spectrum in the L-1(b) band region analyzed in detail. The L-1(b) ECD band of chiral benzene compounds is often used to assign absolute configurations on the basis of sector rules. However, L-1(b) ECD bands of several benzene derivatives are associated with a forbidden character and show marked vibrational progressions strongly modulating their shape. This is also true for compounds 1 and 2, the latter also showing a peculiar thermochromism. The low-temperature ECD spectrum of 2 displays in fact an alternation of positive and negative ECD maxima. Vibronic ECD calculations performed within a TDDFT scheme allowed a full rationalization of the observed ECD spectra of 1 and 2. Especially in the case of 2, the ECD spectrum in the L-1(b) band region results from a complex balance of Franck-Condon and Herzberg-Teller effects, as well as of conformational factors. Therefore, straightforward sector rules cannot be safely used to assign the absolute configuration of even these simple derivatives.