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

Abstract

The alkylbenzene derivatives (R)-PhCH(CH₃)(t)Bu (1) and (R)-PhCH(CH₃)(i)Pr (2) were taken as paradigms of chiral benzene compounds and their vibronic electronic circular dichroism (ECD) spectrum in the (1)L(b) band region analyzed in detail. The (1)L(b) ECD band of chiral benzene compounds is often used to assign absolute configurations on the basis of sector rules. However, (1)L(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 (1)L(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.