Dynamic nuclear polarization to enhance NMR signal strength with electron nuclear double resonance (ENDOR-DNP)

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Nuclear magnetic resonance (NMR) is an amazingly powerful technique for studying everything from drug molecules to working human brains. The first step in a magnetic resonance experiment is to polarize the spins which is like making many tiny compass needles point in the same direction. However, most NMR experiments are slow because of the small fraction of nuclei which are spin polarized. Electrons are much more easily polarized but the analogous technique, electron paramagnetic resonance (EPR), is only useful for studying those materials with unpaired electron spins. We are developing the equipment to efficiently transfer electron spin polarization to nuclear spins, allowing a wide range of exciting NMR measurements that would not otherwise be possible. This transfer process is called dynamic nuclear polarization (DNP).Our DNP equipment uses high magnetic fields of up to 14.1 T. This high field allows more nuclei to be resolved in NMR permitting the study of more interesting samples. Progress with DNP at these magnetic fields has been slow because the corresponding frequency for EPR is 397 GHz, which is in the technologically-difficult THz frequency region. However, we have the equipment to generate, control and detect this frequency for EPR and DNP. Our source of 397 GHz radiation is smaller, more reliable and less expensive than competing technologies, but produces correspondingly less power. The DNP that has been performed in the past uses this power inefficiently so would not be feasible, but we have demonstrated a prototype solution to this problem: by simultaneously driving EPR and NMR in a suitable polarizing agent, highly efficient DNP is possible. This technique is known as electron-nuclear double resonance (ENDOR) and the polarizing agent is a nitrogen atom inside a buckyball: N@C60. Our previous experiments demonstrate the proof-of-principle but we have never tried to use NMR to readout the nuclear polarization. Succeeding in this would show that ENDOR-DNP is useful for NMR and would be an important breakthrough.