| Popis: |
We recently used selective (2)H 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 α and γ (1)H spins on the fluorene rings are responsible for generating a zero quantum (ZQ) mediated positive bulk polarization. Here, we establish that the phenyl (1)H 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-d(5)-BDPA offers >50% improvement in OE-DNP enhancement compared to h(21)-BDPA attaining a maximum of ~ 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. |
