Correlation of the Circular Dichroism of Unsubstituted and Ring - Substituted Chiral Phenylcarbinamines with Their Absolute Configurations

The sign of the 1~ Cotton effects (CEs) at about 250-27D nm of the benzene chromophore of unsubstituted chiral phenylcarbinamines and their salts is determined by vibronic borrowing from the lBb-allowed transition at shorter wavelength. For these chiral phenylcarbinamines and their salts of the same generic absolute configuration, the sign of their lLb CEs is the same. On ring substitution, however, transition moments are induced in the benzene ring bonds adjacent to the attachment bond of the chiral group, resulting in enhanced coupling of the lLb transition with the chiral group. Since the sign of the induced contribution to the lLb CEs is determined by the spectroscopic moment of the ring substituent and its position with respect to the chiral group, the sign of the induced contribution may be predicted to be the same or opposite to that of the vibronic contribution. A reversal of the sign of the lLb CEs with ring substitution is thus the overshadowing of a vibronic contribution by an induced contribution of opposite sign. When both the vibronic and induced contributions have the same sign, the sign of 1Lb CEs is unchanged,