ENDOR Studies in the Conformation of 3,3′- or 3,4′-Polymethylenebiphenyl Anion Radicals.

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The ENDOR spectra have been observed for the anion radicals of 3,3′- and 3,4′-polymethylenebiphenyl with various chain lengths. The hyperfine coupling constants of the ring protons of the 3,3′-derivatives are comparable with those of the reference derivative, m,m′-bitolyl, and the biphenyl ring is considered to take the planar conformation. When the chain length is shortened and the polymethylene chain increases its probability of being held on the biphenyl aromatic plane, strong through-space interactions are observed between the aromatic π and the polymethylene pseudo-π orbital. In 3,4′-polymethylenebiphenyl with shorter chain lengths, the coplanar phenyl rings are twisted out by the polymethylene span and the magnitudes of the spin densities at the ortho-position are reduced by the resonance inhibition between two phenyls. McLachaln's MO calculations predict that the twisting angles between the two phenyls are similar to the value reported for 2,2′-bitolyl anion radical. In contrast to the case of the 2,2′-bitolyl anion radical, however, a conformational change in biphenyl resulting from the delocalization of the unpaired electron is suppressed by a tight lock of the rotation of the phenyl groups in the polymethylene derivatives.
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