Benzene solvent induced shifts of the proton magnetic resonance spectra of some benzene derivatives. Importance of charge effects

The benzene solvent shifts of the proton magnetic resonance spectra of numerous polysubstituted benzene derivatives are measured with respect to cyclohexane as reference solvent. For many of these compounds, an additive scheme can be devised in which the effect of a substituent, X, on the solvent shift, Δ, of a proton is given by Δix so that Δ = Σx, Σi Δix where i = ortho, meta, or para. There is a linear correlation of Δmx or Δpx with Taft's substituent constants σm0 and σp0. It is suggested that charge effects are much more important than steric effects in determining both the magnitude and sign of Δ.