In Silico Design of DNP Polarizing Agents: Can Current Dinitroxides Be Improved?

Numerical calculations of enhancement factors offered by dynamic nuclear polarization in solids under magic angle spinning (DNP-MAS) were performed to determine the optimal EPR parameters for a dinitroxide polarizing agent. We found that the DNP performance of a biradical is more tolerant to the relative orientation of the two nitroxide moieties than previously thought. Generally, any condition in which the g_yy tensor components of both radicals are perpendicular to one another is expected to have near-optimal DNP performance. Our results highlight the important role of the exchange coupling, which can lessen the sensitivity of DNP performance to the inter-radical distance, but also lead to lower enhancements when the number of atoms in the linker becomes less than three. Lastly, the calculations showed that the electron T_1e value should be near 500 μs to yield optimal performance. Importantly, the newest polarizing agents already feature all of the qualities of the optimal polarizing agent, leaving little room for further improvement. Further research into DNP polarizing agents should then target non-nitroxide radicals, as well as improvements in sample formulations to advance high-temperature DNP and limit quenching and reactivity.