The origin of the relative nuclear resonance shifts in monosubstituted benzenes has been investigated. In order to obtain more complete experimental information both C13 and H1 resonance shifts in a variety of aryl-X compounds were measured. The H1 resonances were measured on 5 mole % solutions in cyclohexane to minimize solvent effects; the carbon shifts were obtained from natural abundance C13 resonance measurements in the neat liquid. Unambiguous assignments of both H1 and C13 resonance spectra were made possible with the aid of deuterated compounds. The largest resonance shifts were observed for the carbon atom directly bonded to X. As in the corresponding CH3X compounds, these shifts arise primarily from the inductive and magnetic anisotropy effects of X. Magnetic anisotropy effects of X are also observable in both the C13 and H1 resonances at the ortho position. A very close correspondence between C13 and H1 resonances is observed at the para position, where the primary contribution to the relative shifts arises from resonance effects of X. This implies that the proton resonance responds to the π-electron density on the carbon to which it is bonded, and that under favorable conditions, both H1 and C13 resonance shifts might be employed to obtain information about the π-electron density distribution in aromatic systems. At the meta position the C13 resonance shifts are surprisingly small and uniform, indicating small or negligible inductive effects due to X, and there is no evident correlation with the meta-proton shifts. Both the C13 and H1 shifts at the para position show an approximate correlation with chemical reactivity parameters (Hammett σ constants) but no such correlation exists for the meta-carbon or meta-hydrogen shifts.
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Spiesecke et al. (1961) studied this question.