In all dynamic nuclear polarisation (DNP) experiments in liquids carried out so far the electronic spin belonged to the paramagnetic molecule while the proton was part of the diamagnetic solvent molecule. In this paper the relaxation and DNP of protons inside free radicals in protonfree solvents is studied.
A calculation of the DNP coupling parameter ξ is given for mixed scalar and dipolar intramolecular interaction with simultaneous time dependences by electron spin relaxation, electron spin exchange and molecular motion on the basis of the formalism of ABRAGAM and SOLOMON.
Experimental results at 12.5 kGauß were obtained for the t-butyl-protons in the three radicals tri-t-butyl-phenoxyl (TBP), COPPINGER'S Radical (CR) or Galvinoxyl and di-t-butyl-nitroxide (DBNO). The experimental technique differs from the usual NMR-detection in the use of pulse technique and of a dielectric resonator in the microwave equipment.
DNP enhancements from high positive to high negative values could be observed corresponding to mixed proton-electron-coupling from dominantly scalar to dominantly dipolar interaction. All results can be at least qualitatively explained by the theory of the intramolecular interaction since the essential parameters and constants of the interaction can be measured with other independent methods.
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