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Dynamic proton polarization in several free radical solutions has been studied systematically with different radicals at five widely separated magnetic field strengths and at various temperatures. Normally, in organic solutions the dynamic enhancements of the solvent proton resonances decay with increasing field and decreasing temperature. Scalar proton-electron coupling has only been observed in exceptional cases. The spectral intensity functions describing the experimental behaviour are derived from the dipolar interaction whose time dependence is in the first place due to translational diffusion of individual molecules. In addition, a certain solvation effect has to be taken into account. Strong solvation occurs in solutions of hydroxyl containing solvents with nitroxide radicals. In this case, the dynamic proton polarization appears to be quite different. As an example, solutions of tetramethyl piperidineoxyl in acetic acid have been examined in more detail. The results are explained in terms of hydrogen bridged solvates. The important motional mechanism that modulates the proton-electron interaction switches from “translation” to “rotation”, and electron spin density is temporarily transferred to the solvent molecule.

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