New measurements of the solvent effect on the nitrogen hyperfine coupling constant of di‐tert‐butyl nitroxide are reported. These, together with literature data, are used to test various models for the solvent effect. At the Hu¨ckel level of approximation, aNis a linear function of the applied electric field. Thus various reaction field theories may be considered. The widely used Onsager reaction field does not account for the effects of the more polar solvents or for the differences between polar and nonpolar solvents. The Wertheim and Block–Walker reaction fields are better, especially for very polar solvents. However none of these continuum reaction fields is entirely satisfactory theoretically or experimentally. We propose a dipole–dipole model for polar solvents which is superior to the continuum models. From the dipole–dipole model, we suggest that the quantity mgr;rgr;/Mis a convenient linear parameter for polar solvent effects, the factors being solvent dipole moment, density, and molecular weight. The dipole–dipole model should apply to a wide range of polar solutes. Some special situations are not explained by the model, including hydrogen‐bonding solvents, halogenated aromatics, and solvents with more than one conformation. The temperature dependence of the solvent effect is also considered.
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