POLARIZATION TRANSFER DYNAMICS IN DNP EXPERIMENTS

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Currently, the best polarization mechanism for DNP at 5 Tesla is the thermal mixing effect using 4-amino TEMPO free radical as polarizing agent. 'H-signal enhancements of up to F-=300-400 have been observed after microwave irradiation for 120 seconds. The experiment has to be carried out at low temperatures (T=12-20 K) in order to have sufficiently long nuclear relaxation times. However, at the same time the build up kinetic of the nuclear polarization is rather slow. The time constant was found to be aboutr=230 s at these temperatures. Clearly, it is desirable to have faster transfer mechanisms allowing for better sensitivity. We have studied the thennal mixing transfer kinetics for different TEMPO derivates in order to get a better understanding of the transfer mechanism. We found that thermal mixing with 4hydroxy TEMPO as polarizing agent allows a much faster polarizing rate (,r=41 s) with a comparable signal enhancement to 4-amino TEMPO. Experiments can, therefore, be performed at a higher repetition rate allowing for better S/N per unit time. We also started to perform experiments using other transfer pathways than those used with the currently available polarization mechanisms. Cross polarization under matched Hartmann-Hahn conditions should allow a polarization transfer on the timescale of several gs, the inverse of the hyperfine coupling strength. This type of experiment requires fast phase switching and strong fields on both the EPR and NMR channel and became possible with the newly developed high Q double resonance resonator (see subproject 'An Electron Nuclear Double Resonance Resonatorfor DNP and ENDOR at 5 Tesla').