ESR Studies of the Negative Ions of Biphenyl Derivatives. IV. Conformation of 4,4′-Polymethylenebiphenyl Anion Radicals

Abstract

Abstract The electron spin resonance has been observed for anion radicals of 4,4′-polymethylene bridged biphenyls which have various lengths of the polymethylene chain, from the carbon number of n=11 to that of n=16. The relation between the chain length of the methylene bridge and the ring-proton splitting were studied in order to see how the polymethylene span brings on a strain in the biphenyl ring. The geometrical structures of the sp3-hybrid bond of the β-carbons were investigated on the basis of the observed values of the β-proton splitting in each derivative. In the cases of both pentadeca and hexadeca derivatives, the magnitude of both the ortho- and the meta-ring proton splitting are comparable with those of the 4,4′-diethylbiphenyl anion radical, the open-chain model of the present derivatives. A rotation of the β-methylene sp3-bond is forbidden in this case, however, and the coupling constants of the β-methylene protons no longer exhibit an equivalent value; that is, two of these C–H bonds are placed on a coplanar biphenyl plane, but the others are twisted out by about 30° from the 2pz axis of the ring-carbon atom. When the number of the methylene groups is reduced from thirteen to twelve, the magnitude of the ortho-ring proton coupling constant is greatly reduced. The HMO calculations of the spin density were carried out. The observed tendency of the ring-proton splitting can be qualitatively understood on the assumption that the coplanar biphenyl ring may be modified by twisting the phenyl groups about the central 1-1′ bond rather than by slanting them together against the original aromatic plane. A strong beam of the polymethylene chain also causes a modification of the geometrical structure of the β-methylene sp3 bond and results in a reduction of the β-proton coupling constant to a large extent.