Instrument development towards understanding the spin dynamics of radical heterogeneity and clusters in dynamic nuclear polarization

Dynamic nuclear polarization (DNP) has re-emerged as a method to increase NMR signal by orders of magnitude in the past two decades. The recent developments in high frequency microwave (μw) sources, instrumentation, and radical synthesis have substantially enhanced the NMR signal at standard NMR fields via polarization transfer from unpaired electron spins to their coupled nuclei. Since the re-emergence of DNP, there has been a four-fold increase in publications associated with DNP compared to its initial discovery in 1953. This immense interest in understanding and optimizing the DNP mechanisms has led to numerous biological-, materials-, and imaging-based applications. However, only a limited set of sample formulations result in reliable and significant enhancement of the NMR signal – primarily nitroxide-based radicals in aqueous solvents. Furthermore, with recent technological developments, NMR spectrometers at magnetic fields > 20 T are now operational, where the standard DNP mechanisms governed by continuous wave μw irradiation, namely the solid-effect and cross-effect become less efficient. In order to broaden the scope of sample formulations and develop DNP methods for high magnetic fields, the underlying electron-nuclear spin dynamics must be better understood. However, there are limited instruments that can simultaneously acquire the spin dynamics of both the electron and nuclear spins under identical experimental conditions due to the instrumental challenges associated with acquiring these spin dynamics. Therefore, the aim of this work is to implement developments on our home-built static 194 GHz DNP system to allow for dual NMR and EPR detection, 2-source electron-electron double resonance, and arbitrary waveform generation. With these improvements, we can gain insight into the DNP mechanisms, how experimental conditions affect them, and explain odd phenomenon in experimental results. Here this dual-purpose instrument is specifically used to investigate the impact electron-electron (e-e) interactions have on DNP and the underlying spin dynamics. It has previously been assumed that the radicals containing the unpaired electron necessary for DNP were homogeneously distributed through the sample volume; however, we have found that both the solvents’ propensity to form glass polymorphs and the radical type can significantly alter the distribution of mono-radicals throughout the sample. Subsequently, the heterogeneity and clustering of the electron spins will significantly alter the e-e interactions, and therefore, the DNP enhancement and spin dynamics. We find that many e-e interactions (even if they are relatively weak) can cause a significant reduction in nuclear relaxation due to e-e-n mediated relaxation, which allows for a faster build-up of hyperpolarized NMR signal and can decrease acquisition times