New methods for dynamic nuclear polarization in insulating solids : the Overhauser effect and time domain techniques

Abstract

Dynamic nuclear polarization (DNP) is now established as a powerful technique for improving the sensitivity of NMR signals by several orders of magnitude, enabling otherwise impossible experiments. Unfortunately, the enhancements obtained at high magnetic fields (> 9 T) are only a small fraction of the theoretical limit due to the fact that current DNP mechanisms, including the cross effect and solid effect, utilize continuous wave (CW) microwave irradiation, and scale unfavorably with B0. This has motivated us to develop new DNP methods that do not suffer from the same field dependences. Our first attempt resulted in the observation of the Overhauser effect in insulating solids doped with 1,3-bisdiphenylene-2-phenylallyl (BDPA) or sulfonated-BDPA (SA-BDPA) radical. As opposed to all other CW DNP mechanisms, the enhancement of the OE in insulating solids scales favorably with B0, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. This finding sheds a new light on the seemingly well-understood Overhauser effect. Our second approach is to perform time domain or pulsed DNP, which differs fundamentally from CW DNP, and like CP and INEPT transfers, is in principle independent of B0. In particular, we have investigated the performance of two related pulse sequences including the nuclear orientation via electron spin locking (NOVEL) and integrated solid effect (ISE) at magnetic fields ranging from 0.35 T to 3.35 T. The NOVEL pulse sequence relies on a matching condition between the nuclear Larmor frequency and the electron Rabi frequency, resulting in a fast polarization transfer from electron to protons (hundreds of ns time scale). Furthermore, we showed that adding amplitude modulation to the microwave field, analogous to a ramped CP experiment, led to longer mixing time (ps time scale) but improved the enhancement by a factor of 1.4 to 2. Finally, we implemented a new version of the integrated solid effect (ISE) by modulating the microwave frequency instead of sweeping the B0 which is technically challenging in high field superconducting magnets. In comparison to NOVEL, ISE gives similar DNP enhancement even far below the NOVEL condition. Our study sets the foundation for further development of time domain DNP at high fields.