A model for establishing the ultimate enhancements (A∞) in the low to high magnetic field transfer dynamic nuclear polarization experiment

Abstract

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∞) in this experiment for flow liquid/liquid intermolecular transfer (L2IT). L2IT1H and13C data is obtained for benzene and chloroform in order to test the validity of the model. The ultimate1H and13C DNP enhancements obtained for benzene/TEMPO are −150 and −220, respectively. For a chloroform/TEMPO (L2IT) sample, the ultimate enhancements are close to the1H dipolar (−330) and the13C scalar (+2660) limit, respectively. In the latter case, the observed13C 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 in13C DNP studies is also presented. Alternative methods of sample transfer from the low to high magnetic field are also discussed.