ESR Studies of the Negative Ions of Biphenyl Derivatives. III. Restricted Rotation of Alkyl Group in Some ortho or para Higher Alkylated Biphenyl Anion Radicals

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Abstract
The electron spin resonance has been observed for the anion radicals of 4,4′-diethyl biphenyl, 4,4′-diisopropyl biphenyl, 4,4′-dicyclohexyl biphenyl, 2,2′-diethyl biphenyl, and 2,2′-diiso-propyl biphenyl. Both of the β- and γ-proton coupling constants of alkyl groups have been precisely determined, and the positive temperature dependence has been verified for the β-proton coupling constant of 4,4′-dialkyl biphenyls. The ratio (R) of β-proton coupling constant of the other alkyls to the methyl proton coupling constant has been derived, and the evidence of the restricted rotation has been widely established for each derivative based on the angular dependence of the β-proton coupling constant, which can be expressed as follows: ai β=ρπ, Q=B0+Bcos2θ The minimum R value obtained for 2,2′-diethyl biphenyl enabled us to assume that a rotation of the alkyl group would be tightly locked at the ortho position. The R value of 2,2′-diisopropyl biphenyl revealed that the isopropyl β-proton would lie on the biphenyl aromatic plane. The anomalous R values associated with the ortho-alkylated biphenyls can be well understood as resulting from the facts that anion radicals have a more dominant character than do neutral molecules to reduce a twisting angle between the two phenyls, and that the free rotation of the alkyl group is strongly restrained by the steric hindrance between the alkyl group and the closely-oriented ortho-ring proton.