TY - JOUR
T1 - 1H Thermal Mixing Dynamic Nuclear Polarization with BDPA as Polarizing Agents
AU - Li, Yuanxin
AU - Equbal, Asif
AU - Tabassum, Tarnuma
AU - Han, Songi
N1 - Publisher Copyright:
© 2020 American Chemical Society.
PY - 2020/11/5
Y1 - 2020/11/5
N2 - Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the 1H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.
AB - Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the 1H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.
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U2 - 10.1021/acs.jpclett.0c01721
DO - 10.1021/acs.jpclett.0c01721
M3 - Article
C2 - 33058676
AN - SCOPUS:85095799308
SN - 1948-7185
VL - 11
SP - 9195
EP - 9202
JO - Journal of Physical Chemistry Letters
JF - Journal of Physical Chemistry Letters
IS - 21
ER -