Solvent effects in proton and13C N.M.R. chemical shifts of polar compounds

The effects of dilution in various organic solvents on the proton and 13C chemical shifts (at natural abundance) of the methyl group have been measured for the polar solutes methyl iodide and acetonitrile. The results are discussed in terms of the distortion of the solute molecule by its electrostatic interaction with the surrounding medium (reaction field effect). Using an expression for the reaction field derived from the spherical cavity model of Onsager, it is found that the results cannot be accounted for by inserting the static dielectric constants ε for the solvents. The results are, however, consistent with effective dielectric constants intermediate between the static values ε and the optical values n 2; this suggests that the correlation times for random re-orientation of the solute molecules are comparable with the dielectric relaxation times of some of the solvents. Dielectric saturation in the immediate neighbourhood of a solute molecule must also be considered as reducing the effective dielectric constant.