Directly vs Indirectly Enhanced 13C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems

A study of dynamic nuclear polarization (DNP) in polyethylene glycol and related nonionic surfactants is presented. In these experiments, we found the surprising result that DNP enhanced C-13 magic angle spinning (MAS) spectra display two sets of resonances, one with broad and one with sharp spectral features that are 180 degrees opposite in phase. These two sets indicate the presence of a direct polarization transfer channel as expected for C-13 MAS experiments, and a second unexpected indirect polarization transfer channel. Plots of DNP enhancements as a function of applied magnetic field for the two resonances show a superposition of two DNP enhancement profiles for AMUpol in the nonionic surfactant C10E6. The indirect polarization channel can be suppressed by application of a string of H-1 180 degrees pulses during C-13 DNP buildup. The presence of direct and indirect polarization channels is observed in a total of four different nonionic surfactants and with three different radicals, showing that these concurring polarization mechanisms are of general nature. Therefore, the presented findings, including the demonstration of how the indirect polarization channel can be suppressed, are of high importance for all future applications of direct C-13 MAS DNP.