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

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:aiβ=ρπ , Q=B0+Bcos2θThe 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 ar...