SOLVENT EFFECTS ON THE PROTON RESONANCE SPECTRA OF THE DICHLORO- AND DIBROMO-ETHYLENES: REACTION FIELD EFFECTS IN ALIPHATIC SOLVENTS AND SPECIFIC INTERACTIONS IN AROMATIC SOLVENTS

The proton resonance shifts relative to internal cyclohexane and tetramethylsilane of cis and trans dichloro- and dibromo-ethylenes were measured in dioxane–water solutions, aliphatic solvents, and aromatic solvents. The dielectric constants of the dioxane–water solutions could be smoothly varied between 2 and 10 and, by taking Δ = cis–trans shifts as containing only the dipolar reaction field effect, the data for these solutions were treated as a test of the Buckingham reaction field equation. Linear plots were obtained and reasonable values of the coefficients of the E term confirmed the usefulness of this equation. An equation based on a nonspherical cavity was also used. It was also shown that quadrupolar reaction fields affected the proton shifts. Sizeable deviations from the dioxane–water standards were found for halogenated aliphatic solvents and those containing double or triple bonds. These deviations are interpreted in terms of the shape of the solute molecules and dispersion and anisotropic interactions. The much larger deviations found for aromatic solvents are harder to rationalize but specific interactions were found for both cis and trans forms, indicating an inadequacy in a simple dipole-induced dipole model.