Comparative Study of the Magnetic Field Dependent Signal Enhancement in Solid-State Dynamic Nuclear Polarization Experiments

A detailed study of the magnetic field dependent signal enhancement in solid-state dynamic nuclear polarization (DNP) experiments is presented for a specific sample consisting of AMUPol dissolved in the nonionic surfactant C10E6. C10E6 displays a superposition of “direct” and “indirect channel” resonances in 13C MAS DNP NMR spectra. The shapes of the DNP enhancement profiles are essentially identical for the 1H MAS, 1H → 13C CP MAS, and 13C MAS indirect channel signals, which confirms that the same polarization transfer process from electron to proton is responsible for the obtained enhancements of these experiments. The shape of the DNP enhancement profiles of 1H and of 13C direct channel resonances reveals that the cross effect is the dominant polarization transfer mechanism for the studied sample. The magnitudes of the 13C MAS DNP enhancement profiles for 1H → 13C CP MAS, direct and indirect channel signals were found to be not uniform. For 1H → 13C CP MAS and the indirect channel signals, this observation is related to relaxation effects of the methyl group carbon. For the 13C MAS direct channel resonances, differences in magnitudes are discussed in terms of preferential structural orientation of the polar ethylene oxide headgroup of C10E6 toward the AMUPol radical.