Three-Spin Effect in Dynamic Nuclear Polarization: Quantitative Determination of Internuclear Interactions

The three-spin effect in dynamic nuclear polarization has been quantitatively investigated. The effect, arising from internuclear transfer of polarization, is observable only in carefully chosen dilute free-radical solutions. Test samples consisted of DPPH in hexafluorobenzene mixed with either normal or perdeuterated benzene. Theoretical calculations support the experimental result that the three-spin effect adds 24 units to the enhancement for a radical concentration of 0.6 × 1017 spin/cc. The effect has direct relevance in the case of chemically induced dynamic nuclear polarization where the enhancement is small.