Positive enhancement of proton resonances by dynamic nuclear polarization

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Abstract
The positive Overhauser enhancement observed for the tert-butyl groups of tri-tert-butyl phenol in solutions of tri-tert-butyl phenoxyl radical has been shown to be associated with the proton exchange between the radical and phenol molecules. It is postulated that in the radical molecule the scalar coupling between the electron and the tert-butyl protons occurs at least in part by a hyperconjugative mechanism and therefore the coupling is modulated by random rotation of the tert-butyl groups. This leads to a positive polarization of the tert-butyl protons, which persists when the radical accepts a proton and becomes a tri-tert-butyl phenol molecule.