Isotropic Hyperfine Interactions Drive Cross-Effect Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) is a powerful route for overcoming the inherent sensitivity limitation of solid-state nuclear magnetic resonance (ssNMR) spectroscopy by transferring high electron spin polarization to surrounding nuclear spins. Cross-effect (CE) DNP is the most efficient mechanism in solids. CE requires several conditions to be met, primarily the presence of two coupled electron spins with resonance frequencies separated by the nuclear Larmor frequency. This condition is typically achieved through the presence of large anisotropic spin interactions, which shift the transition frequencies of the two coupled electron spins with respect to each other. Here we present an alternative approach, where the CE condition is met via isotropic interactions. This is advantageous as it makes CE independent of the sample orientation, thus making the enhancements independent of the MAS frequency and enabling the use of fast relaxing polarizing agents. We demonstrate the feasibility of the approach in experiments and simulations for Mn(II) dopants as polarizing agents, making use of the isotropic hyperfine interactions with its ⁵⁵Mn nuclear spin to achieve the required frequency difference.