Amplified Overhauser DNP with Selective Deuteration: Attenuation of Double-Quantum Cross-Relaxation

We recently used selective 2H labeling of BDPA to investigate the Overhauser Effect (OE) dynamic nuclear polarization (DNP) mechanism in insulating solids doped with 1,3-bis(diphenylene)-2-phenylallyl (BDPA), and established that the alpha and gamma 1H spins on the fluorene rings are responsible for generating a zero quantum (ZQ) mediated positive bulk polarization. Here, we establish that the phenyl 1H spins relax via double-quantum (DQ) processes and therefore contribute negative enhancements which attenuate the OE-DNP. With measurements at different magnetic field strengths, we show that phenyl-d5-BDPA offers >50% improvement in OE-DNP enhancement compared to h21-BDPA attaining a maximum of similar to 90 at 14.1 T and 5 kHz MAS, the highest observed OE-DNP enhancement to date under these conditions. The approach may be utilized to optimize other polarizing agents exhibiting an OE, an important DNP mechanism with a favorable field and spinning frequency dependence.

Automated summary

In this study, selective 2H labeling of BDPA was utilized to investigate the dynamic nuclear polarization (DNP) mechanism of the Overhauser Effect (OE) in insulating solids. It was found that the alpha and gamma 1H spins on the fluorene rings were responsible for generating a positive bulk polarization, while the phenyl 1H spins contributed negative enhancements that attenuated the OE-DNP. Phenyl-d5-BDPA showed over 50% improvement in OE-DNP enhancement compared to h21-BDPA, achieving a maximum of approximately 90 at 14.1 T and 5 kHz MAS.