HIGH FIELD DNP & ELECTRON CROSS RELAXATION IN INHOMOGENEOUSLY BROADENED EPR LINE

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We have presented a model for analyzing polarization transfer via the thermal mixing dynamic nuclear polarization (DNP) mechanism at high magnetic field strengths that accounts for electron-electron cross-relaxation within an inhomogeneously broadened EPR line and have applied this model to simulate DNP and ELDOR experiments in the 4-amino TEMPO/water/glycerol system. These studies indicate that the 40 mM 4-amino TEMPO EPR line is in the intermediate cross-relaxation regime. Thus, the three-bath thermal mixing model, used previously to describe low field experiments, does not adequately explain the.polarization dynamics of the system at high fields. Given some basic parameters, such as the EPR line shape and spinlattice relaxation times, the model developed here can be used to predict the efficiency of DNP at even higher magnetic field strengths, where thermal mixing will be more severely attenuated due to the greater spectral dispersion and hence slower cross-relax ation rate , and with polarization agents other than the 4-amino TEMPO radical. Using this approach, it may be possible to develop optimal paramagnetic systems for performing DNP under experimental conditions suited for solid-state NMR studies of biological samples.