A Model for Establishing the Ultimate Enhancements (A infinity) in the Low to High Magnetic Field Transfer Dynamic Nuclear Polarization Experiment

In a preliminary report, we have demonstrated transfer of a flowing bolus enhanced in low magnetic fields (e.g., 0.33 T) with dynamic nuclear polarization (DNP), but monitored in a high magnetic field (4.7 T). The advantages of the high magnetic field monitoring approach include: 1) greater chemical shift dispersion, and 2) improved signal strength in comparison with static low field DNP experiments. In the present study, a model is developed to predict ultimate DNP enhancements (A infinity) in this experiment for flow liquid/liquid intermolecular transfer (L(2)IT). L(2)IT (1)H and (13)C data is obtained for benzene and chloroform in order to test the validity of the model. The ultimate (1)H and (13)C DNP enhancements obtained for benzene/TEMPO are -150 and -220, respectively. For a chloroform/TEMPO (L(2)IT) sample, the ultimate enhancements ate close to the (1)H dipolar (-330) and the (13)C scalar (+2660) limit, respectively. In the latter case, the observed (13)C DNP enhancement exceeds the thermal Boltzmann magnetization at 4.7 T by a factor of 21. For a 1-chlorobutane/TEMPO sample selective enhancements were observed at different sites in the molecule. For example, the C-1 carbon exhibits a large scalar enhancement, whereas, the other carbons exhibit dipolar enhancements. Data illustrating the importance of three-spin effects in (13)C DNP studies is also presented. Alternative methods of sample transfer from the low to high magnetic field are also discussed.