Dynamic Nuclear Polarization Using Electron Spin Cluster

Dynamic nuclear polarization (DNP) utilizing narrow-line electron spin clusters (ESCs) to achieve nuclear spin resonance matching (ESC-DNP) by microwave irradiation is a promising way to achieve NMR signal enhancements with a wide design scope requiring low microwave power at high magnetic field. Here we present the design for a trityl-based tetra-radical (TetraTrityl) to achieve DNP for ¹H NMR at 7 T, supported by experimental data and quantum mechanical simulations. A slow-relaxing (T_1e ≈ 1 ms) 4-ESC is found to require at least two electron spin pairs at <8 Å e-e spin distance to yield ¹H ESC-DNP enhancement, while squeezing the rest of the e-e spin distances to <12 Å results in optimal ¹H ESC-DNP enhancements. Fast-relaxing ESCs (T_1e ≈ 10 μs) are found to require a weakly coupled narrow-line radical (sensitizer) to extract polarization from the ESC. These results provide design principles for achieving a power-efficient DNP at high field via ESC-DNP.