From The DNP-NMR Blog:
Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization #DNPNMR
Equbal, Asif, Kan Tagami, and Songi Han. “Balancing Dipolar and Exchange Coupling in Biradicals to Maximize Cross Effect Dynamic Nuclear Polarization.” Physical Chemistry Chemical Physics 22, no. 24 (2020): 13569–79.
https://doi.org/10.1039/D0CP02051F.
Dynamic nuclear polarization (DNP) by the Cross Eect (CE) has become a game changer for solid-state nuclear magnetic resonance (NMR) spectroscopy. The eciency of CE-DNP depends on the strength of the electron-electron coupling in biradical polarizing agents. Hence, the focus lately has been on designing biradicals with a large net exchange (J) and dipolar (D) coupling. In this study, we reveal that the crucial factor for CE-DNP is not the sum, J+D, but rather the relative magnitude of J and D, expressed as the J/D ratio. We show that the mechanistic basis of this interference lies in the isotropic v.s. the anisotropic nature of the J and D couplings, respectively. This interference can lead to a small (eective) electron-electron coupling for many orientations even when J+D is large, resulting in non-adiabatic rotor-events. We find that when 0< jJ/Dj < 1 the CE-DNP eciency is attenuated for the majority of orientations, with greater attenuation observed at higher magnetic elds and faster Magic-Angle Spinning (MAS) frequency. The interference eect of J and D coupling introduced in this study can explain why many biradicals with high or comparable J + D still show signicantly divergent DNP performances. We debut J/D as a consequential criteria for designing ecient biradicals to robustly perform across a large range of B0 elds and MAS frequencies.
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