Conformation and Optical Rotation of Restricted Biphenyls. Configurational Correlation of Biaryls by Optical Displacement. The Absolute Configuration of Restricted 1,1'-Binaphthyls¹

Electronic spectra, optical rotatory power, and absolute configuration of the 1,1?-bianthryls

“…In the Huckel theory the red-shift of the p-band is the same for the bianthryls (1) and ( 11), and on the assumption that the resonance integral p of the internuclear bond is governed by the relationship,21 p = po cos2 a (14) the calculated red-shift is 750cm-1. This value for the conjugative red-shift is probably of the right order, the interaction between the two excitation dipoles contributing an additional red-shift which is larger for the bridged bianthryl (11) than for the bis-ester (I).…”

“…4). The red-shift is larger for the bridged bianthryl (11) than for the bis-ester (I), since the dihedral angle a (fig. 1) between the planes of the anthracene nuclei, and, thus, the distance between the two excitation dipoles, is the smaller in the former compound.…”

“…The observation that the bianthryls (I) and (11) give, near 2500A, two circular dichroism bands with opposed signs and approximately equal areas and a split absorption band (fig. 2 and 3) indicates that the P-band of anthracene is due to an excitation with long-axis polarization, as required by the molecular-orbital theory,lg and as demonstrated previously by studies 14 of the crystal spectrum of anthracene.…”

“…Accordingly, thep-band of the bianthryls (I) and ( 11) is ascribed to a transition with B symmetry, arising from excitations which are directed along the short axis of each of the two constituent anthracene nuclei, ELECTRONIC SPECTRA OF 1,1 '-BIANTHRYLS Relative to that of anthracene, the p-band of the bianthryls (I) and ( 11) is shifted to the red, the shift being larger (tabie 1) for the bridged compound (11) than for the bis-ester (I). In part, the red-shift is explained by the low-energy head-to-tail orientation of the excitation dipoles of the two anthracene nuclei in the corresponding B-transition of 1,l'-bianthryl (fig.…”

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The electronic absorption and circular dichroism spectra of the (-)-isomers of the bianthryl bis-ester (I) and the bridged bianthryl (11) have been analyzed by means of a coupled oscillator model and molecular orbital theory. It is shown that the (-)-isomers of the bianthryls (I) and 01) have the particular absolute configuration in which the two anthracene nuclei assume the form of a righthanded two-bladed propeller when viewed along the two-fold axis of the molecule, and that the dihedral angle between the planes of the anthracene nuclei is 110" in (I) and 70" in @I).

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