Nuclear Cross-Effect NMR

Dynamic nuclear polarization (DNP) has significantly enhanced the sensitivity of nuclear magnetic resonance (NMR) experiments. While Overhauser DNP has a well-established nuclear counterpart, the nuclear Overhauser effect (NOE), the cross-effect DNP has traditionally been considered as an exclusively electron-mediated process. Here, we present the first evidence of a nuclear counterpart to the cross effect, termed the nuclear cross effect, which mediates polarization transfer in a three-spin nuclear system. The effect was experimentally verified using single-crystal ¹³C-¹⁵N-labeled glycine, with results consistent with both analytical theory and numerical simulations. The experimental realization of the pulsed nuclear cross-effect in this work provides new insights into its theoretical foundations, addressing challenges posed by hardware limitations and the absence of a suitable model system for pulsed cross-effect DNP. Furthermore, the pulsed cross-effect mechanism has the potential to advance enhanced NMR applications in various fields.