Carbon-13 Nuclear Magnetic Resonance Studies of Biphenyl Derivatives II. FT–NMR of Halobiphenyls

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The natural-abundance 13C NMR spectra of the ortho- or the para-halobiphenyls were obtained by the pulse Fourier transform NMR (FT-NMR). For the para-derivatives, the observed chemical shifts of the C1 C2, and C3-positions gave an excellent linear dependence on the electronegativity of the substituents. The dependence for the C4-position, which showed a deviation from the linear plot, may be due to the heavy atom effects. The additivity of the chemical shifts holds also for the ortho-derivatives except for the cases of the bridgehead carbons, the C1 and C1′-positions. In particular, the observed chemical shifts of the C2, C3, and C4 positions for 2,2′,6,6′-tetrachlorobiphenyl showed a good agreement with the predicted values, whereas a large discrepancy can be seen between the observed and the assumed values for the C1 and C1′ positions. The effects on the chemical shifts due to the steric hindrance are discussed by comparison with the results of the hindered alkylbiphenyls. The temperature dependence of the spin-lattice relaxation time was investigated for 4,4′- and 2,2′-dichlorobiphenyls. The activation energies of the molecular reorientation are negligibly small at the C1 and C4-positions for the para-derivative, but the ortho-derivatives have higher values at these positions, indicating anisotropy for the molecular motion.
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