Heteronuclear Spin Decoupling under Cryogenic Magic-Angle-Spinning Dynamic Nuclear Polarization Conditions

Heteronuclear decoupling is an essential element in the solid-state magic-angle spinning (MAS) Dynamic nuclear polarization (DNP) experiments involving low gamma (γ) nuclei like 13C and 15N coupled to protons. Under the constraints of limited (< 100 kHz) maximum radio frequency (RF) amplitude and intermediate (30-45 kHz) spinning frequencies, imposed by the cryogenic DNP setup, the typically utilized heteronuclear decoupling schemes like TPPM and SPINAL-64 suffer from near rotary-resonance recoupling (R3). Here, we demonstrate the effectiveness of the refocused continuous-wave (rCW) decoupling sequence for heteronuclear 1H decoupling near the R3 regime under DNP conditions. We compare the performance of rCWApA decoupling with the widely used SPINAL-64 scheme and show a significant improvement in the heteronuclear decoupling efficiency, confirmed through T2' evaluations. The superior decoupling efficiency of rCWApA stems from its half-wave symmetry in the RF interaction frame, which enables effective decoupling of first and second-order effective heteronuclear coupling Hamiltonian, even at low RF amplitude levels and hence minimizing R3 effects better than any other decoupling scheme.